The sub monolith token allows a player expand the build range (Sphere of influence) of an existing settlement. Sub monoliths must be placed at the periphery, outside the build range of the settlement. More than one sub monolith can be placed around a settlement.
They can be used both to increase the build range of a settlement, or to create new mini settlements in other areas.
Sub monoliths look like common monoliths but their icon has 2 more dots on each side of the arc.
A settlement will not be entirely destroyed if at least a sub monolith will remain.