Each separable part of a Fragment must have an associated collision object in order for physics to work. This means each bone with Use Physics enabled should also have a collision object linked to it.
You link collisions to bones the same way you link Drawable Models to bones as described in Drawables (.ydr) > Rigging.
Each collision object under a Fragment also has a Physics subpanel where you can define its mass.
There is also a tool for automatically calculating mass based on the collision material's density and the object's volume. Navigate to Sollumz Tools > Fragment > Set Mass. There you will find the Calculate Collision Mass tool.
In order for this tool to work, the collision object must have a collision material added.
Volume and Inertia are essential physics properties for collisions. You can find these options under Object Properties > Sollumz.
Typically when creating completely custom collisions, you would auto-calculate these values during export, as calculating these involves a lot of math relating to the dimensions and shape of your collisions.
