Jonas:
The circle symbol indicates a dynamic dimension point (the dimension string will follow changes to that point on the element), the square symbols indicate a static point (not associated to an element point). For example, a circle would appear at the end of a wall, a square at the wall's midpoint.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David