When deleting and rebuilding the index (in that case native fulltext search), the "Total number of indexed words:" stays the same before and after. The "Total number of word to post relations indexed:" decreases.

Prior to deleting and rebuilding the index I was able to search (and find) a specific word and got all threads listed where the searchword was in the topic or in the text.

After deleting and rebuilding the index the search results only contained threads where the searchword was in the text.

Obviously rebuilding the index does index both topics and text, but the word to post relation index only contains references to word in the text.