On selecting contacts you are presented with the contact filter with types as well as various other lookup options. Selecting and deselecting contact types will restrict your search to only the selected contact types. When there is a cross next to a contact type, that means that once it has been filtered that contact type will not show. When there is a tick next to a contact type, that means that once it has been filtered that contact type will show. And when it is blank next to a contact type, that means that once it has been filtered it is indifferent to that contact type. (Default: All on except Archived)

Typing in the filter will filter to show only contacts where the field contains the letters selected.

Some fields like Debtor will find a single account based on you capturing the code or name and here you must select one from the filtered list to use it.
i.e. A search on First Name for “oh” will return John, Ohana and Shyloh.