AdvancedSearch aims to make important existing keywords accessible to people who don't know them by heart. In theory, we would like to support category search and prefix search (often used for searching in subpages for a page). However, for the MVP both of them come with too many implications:
Category search makes most sense to be done as deep category search (i.e. also searching through subpages of a specified category). This is currently not implemented in the usual search.
Prefix search only allows the specification of one or all namespaces. With the current interface, this would result in unexpected side results influencing the whole search, especially if people have specified namespaces in the corresponding field.
While future versions should include both options, we decided to take them out of the MVP.
Remove the fields "Search in category" and "Subpages of that page" from the AdvancedSearch.