In Importacular, a Yes/No (or True/False) field can be set to three different settings:

Yes/True - The box should be ticked/checked (see below).

No/False - The box should be unticked/unchecked (see below).

Neutral - This shows a box within a box (see below). This setting is the default option.

One use for a neutral is for the Preferred Address field.  When an address should be marked preferred for new constituents, but not for existing, the box should be left in a neutral state.