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Hit the button and choose which settlement to send them to - as soon as their name changes to Provisioner you'll be able to share resources. Supply caravans mean that connected settlements pool their Workshop contents - enter Workshop mode, highlight a spare Settler, and you'll see an option for Supply Line pop up at the bottom of the screen.
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That might not mean much, but let me ask you - have you ever tried to build a Radio Transmitter and been told you don't have a crystal, because you stored it in a workshop 50 miles away? This solves the inevitable rage-destruction. Grab the first rank of the Local Leader perk (it's in the Charisma section) and you'll be able to send supply caravans from one settlement to another. If you're going to spend time building a sizeable settlement this is, if not essential, then really, really, really, very, really helpful. Seriously, how the hell did they make these?
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