X4: Foundations – Capturing Larger Ships
L and XL ships and stations need to be boarded to be captured. Boarding requires sufficient Marine crew on the playership. To prepare for boarding, the target needs to be nullified (taking out the engines, turrets and shields). To initiate boarding, select the playership and then right click the target and order Marines to board the target.
Try to keep the subsystems destroyed, and work on reducing hull levels but not to the extent that the boarding marines will destroy the ship.
Once the target hull is sufficiency low to reduce boarding resistance below the boarding strength of the players forces (20 or more is good) the ship is as good as captured. If successful, a Marine can be assigned as the captain for the ship or the player can move to the ship personally.