A twin airport is used where a base has two airport that are close, and where crew can check in at both airport and check out from the twin. This is used by airlines where crew transport themselves to the airport by public transport or by a company taxi service.
The connection doesn't require ground transportation and it will not create a legality warning.
On the roster it is indicated with a green line (default color).
The color can be customised from the setup parameter "GanttColors"