Recently did my wife's ILR application but I'm sure they'll be same or similar. The categories were things like Finances, Education, Life Events, Sponsor/employment, etc.
Most are pretty obvious but on the ones where I wasn't sure or I felt it could go under more than one category I uploaded the same document to multiple categories "just in case". There are not that many so wherever you upload to they'll find it easily enough
Make sure your files are less than 6mb though or you won't be able to upload them.