Game News: Fallout 4: How Settlement Supply Lines Work
A supply line is something that connects settlements, and allows them to share workshop inventory space
you need to have the first rank, of the Local Leader perk to establish supply lines, meaning you have to have six Charisma
The supply line can be a connect-the-dots-esque chain, and will work to connect everything perfectly, Also, since the settlers assigned to the supply lines, will actually be wandering the wastes it’s a good idea, to give them some decent weapons and armour
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