Supply line

In Fallout 4, supply lines can be created between settlements to share resources and inventory among them.

Requirements for creating supply lines:
 * At least two owned settlements
 * At least one free settler
 * The first rank of the Local Leader perk
 * The two settlements must not already be (directly) connected with a supply line

Benefits
Linking the settlements with supply lines provides the following benefits:
 * When building at a workshop or using a crafting station inside a linked settlement, one can use the junk, aid and miscellaneous items in the inventory of all linked workshops from all linked settlements (junk does not have to be salvaged first).
 * Armor and weapons in the workshop are exclusive to the settlement. What is in the workshop inventory passes to that specific settlement's armor and weapon workbenches.
 * Excess food and water will be used to supply all the settlements in the supply network. How a "green" shortage affects happiness in a settlement is not covered here.
 * In the Pip-Boy "Workshop" menu, the game will show actual shortages for individual settlements with warning triangles, which causes unhappiness, but if the category is green, then the network is supplying the shortage.
 * Provisioners can double as friendly patrols, and will engage enemies that obstruct their paths. An abundance of supply lines intersecting the various settlements can make traveling across the Commonwealth much safer, particularly if the provisioner is equipped with proper weapons and armor to deal with the kinds of enemies that they might encounter on their route.
 * It is possible to establish supply routes between the Commonwealth and the Island, giving one access to stored resources in both places.
 * If a provisioner is encountered in the wasteland, they can be given items that they will carry to the settlement they are assigned to. Thus, provisioners can be used as "personal movers" to clear up inventory space.

Usage
To create a supply line, use the workshop in one of the settlements, highlight a settler and press [Q] (or RB/R1 on XBOX/PS4) to display a list of owned settlements. Select the settlement to which the player character wants to link the current settlement.

To review existing supply lines, open the world map and press [C] (or L1/LB on PS4/XBOX) to view all the settlements connected to the supply line network.

Settlers assigned to supply lines will become provisioners and will travel between settlements (usually leading a pack-brahmin). To remove the supply line, simply reassign the provisioner to some other work object in their settlement.

Carefully planned out routing of the supply lines is in order. Provisioners will attempt to follow roads if at all possible. This can be problematic as there are several locations which take the provisioner past or through static enemy locations such as the Scrap Palace or through enemy spawning points such as the road directly east of the Starlight Drive-In. A severe problem location is the road in front of the Medford Memorial Hospital - Malden Center - Slocum's Joe Corporate Headquarters and Med Tek Research which has several static enemy locations as well as spawn points. This effectively cuts off the eastern settlements from the west. A result of attacks upon provisioners are unaccompanied pack brahmins wandering all over the map.The provisioner will recover from the attack and continue on it's assigned route while the brahmin is left to it's own devices. The may eventually result in the raining brahmin phenomenon where upon entering a cell the game doesn't have a location to assign the brahmin and simply places it at random on or above the terrain.
 * The provisioner will actually travel between the two connected settlements, so it may be difficult to reassign the worker, since they may not be in their home settlement. It is also possible to reassign the provisioner using the Vault-Tec Population Management System that came with the add-on Vault-Tec Workshop.
 * Provisioners still count towards the population limit of their respective settlements, as well as food/water requirements, despite hardly ever being in the settlement.
 * Settlements share resources with all other settlements that are part of its supply line network. Two settlements do not have to be directly connected. In other words, if the Sole Survivor connects settlement A → B, B → C and C → D, then A, B, C and D will also share resources with each other despite not directly being connected since they are still part of the same network.
 * Provisioners will engage hostile creatures and enemies along their path, so it is advisable to equip them with appropriate weapons and armor so they can defend themselves.
 * Like companions, provisioners cannot die by enemy fire; with enough damage, provisioners will be briefly knocked down, but they will get back up after a while. They will, however, die from damage taken from the Sole Survivor's mines.
 * To change or update the provisioner's clothing or weapons, initiate dialogue and trade items with them.
 * Creating a supply network with two overlapping circles (A → B → C → D → A and A → C) can make some of the settlements have their food/water set to 0. To fix this, one needs to remove one of the circles and then activate the workshop at the affected settlements to update their resources.

Bugs

 * When setting up a supply line, only settlements which are at least one settler under the population limit can be selected as a target. This may be a possible bug, since provisioners do not change the population of the targeted settlement.
 * Provisioners that were moved to the destination of their supply lines may keep the title of provisioner and their backpack appearance. The provisioner can be assigned any regular settler task and the supply line indeed ceases to exist.
 * Supply caravans and their respective provisioners can become stuck on their routes or just disappear despite the Pip-Boy still showing a supply line connected between two settlements. This can make it extremely difficult or impossible to make any changes to your supply lines after they have been established.
 * Possibly, the missing provisioner may be pinned down in combat. Arriving and 'rescuing' the provisioner will re-establish the supply line.
 * Go to a different settlement, relocate someone to the affected settlement. After confirming, immediately assign him/her as a provisioner to the other affected settlement. Make sure to do this as quickly as you can since there will be a delay before the system updates the list of destination settlements. Confirm assignment and allow the system to update, then immediately reassign the settler to another task like farming.
 * When a provisioner is assigned a new supply route or another task or is moved to another settlement while they are visiting another settlement, the old supply route will remain active in the Pip-Boy supply lines map, and the resources from the previous route will still be available to the provisioners original settlement.
 * If an Automatron is created in settlement "A", moved to settlement "B" and immediately made a supply route from B to A (via "A" workshop mode) it can cause some bugs in settlement "A" like wrong numbers of settlement resources (food/water/people) or a permanent supply line.
 * A possible fix may be: fast travel to "B" settlement, find the bugged automatron and command it via workshop mode to do something else (like farming). While still in workshop mode, open the console, enter the command and close the console.  Press Q on the newly spawned settler, assign it to the "A" settlement and set their home in the "B" settlement so to make them take the broken supply line route. Finally, assign the spawned settler to do something else to remove it from traveling. This should fix the broken supply line. The broken automatron can be reassigned to the same route.