It is possible to exclude a crew member assigned to 'Line Training' / 'Line Check' from the basic crew assigned to the flight.
Two below option is available in the 'Settings' > 'General Settings' > 'Crew planning' tab:
Once the option is selected, Crew on the 'Line Training' / 'Line Check' will affect the color of the square indicating the crew composition in the 'Crew Timeline'.
As presented in the example, the aircraft is set to a minimum cockpit crew of 2. On the first flight, one of the crew is assigned to the 'Line Check' duty (indicated by the purple triangle) therefore the first flight is highlighted yellow, indicating the partial crew assignment.
The second flight is highlighted green, since there is no training assigned.