Trying to wrap my head around rest length in dops. What does changing this do for constraints? Thanks in advance.

It's the length between two anchor points that define a constraint polyline. That length will automatically measure by `perimeter` if you use `convert line` or `connect adjacent pieces` node.

Say Box A to Box B constraint by each centroid, their position separate by a space of 1 meter, get constraint by their centroid, rest length from `convert line` or `connect adjacent pieces" is automatic measure then nothing happen while simulation. But if you alter restlength=0, both box A and box B will pull to each other because there was no length between them anymore, so `rest` is where they were initially created. They will push far from each other if those length values alter longer than their initially created.