Technically no table stores that data, although you could say it's a combination of evo_postcats and evo_categories.

We think of posts as being "in a blog with a category (or multiple cats)", but the database actually stores posts as "attached to a category which happens to be attached to a blog".

So your post gets added to evo_posts with a unique post_ID and a main_cat_ID. It also gets added to the evo_postcats table with an entry showing the post_ID and the cat_ID for the main category and any subcats you may have selected. Each category is listed in evo_categories and shows a blog_ID for that cat. If it's a subcat it also shows the parent category in that table.

This way your posts can be in multiple blogs by enabling the feature that lets you post across multiple blogs. You can also move an entire category's worth of posts from one blog to another by enabling that feature. Both of these would require editing one of the files in your conf/ folder, but I don't recall off the top of my head which file it is.