Writer master document 'categories' index?

On, for example, a WordPress blog, I can assign each post to a category or several categories and I get a nice index of categories I can later use to navigate the posts. I’ve been trying to work out how to do this with an index in a master document (with subdocuments taking the place of posts), but I can’t find that it’s possible. Am I missing something or is it really not possible?

My apologies.

After scratching my head in puzzlement over the manual for about an hour, then posting that question on here, I made myself a cup of tea and as I was doing so the answer hit me in a hot flush of embarrassment and facepalms. All I have to do is to write the words I want to use as categories at the bottoms of the subdocuments and then index them, don’t I? Never thought of putting them actually IN the subdocuments …

Apologies again. I’ll leave the question and my answer here, just in case anyone else is as stupid as me.