The optional complex category search form allows you to refine your category search results by removing stories with undesired categories from the stories returned.
In the complex form, the + adds a category to the search and the - removes a category which means for example:
if you check the + mark for MF mf first
and then you check the - for the incest category
then the system will find all the stories that have MF mf first and don't have the incest category. So the minus works like the category exclusion preference but on one search.