From The Foundry MODO SDK wiki
Categories can describe a the following:
- Form Categories and Groups, which provides a way to insert one form at the head or tail of a second form without modifying that second form in any way. This is primarily used by plug-ins to add their forms to Item Properties, the menu bar, and other forms in modo. The normal act of parenting a form to another form causes the parent form to be changed, and is not suitable when creating a plug-in for distribution.