Cannot create a revision

SOLVED

Some inventory assemblies will allow the creation of new revisions, but other assemblies will not. The "Revisions" button at the bottom of the "Bill of Materials" tab is greyed out and not active. I have searched help to see if there is a setting somewhere that allows some assemblies to have revisions, but I cannot find anything. Any ideas? As an aside, the whole category and subcategory thing below are confusing and non-helpful.