@redrum It is a unit property. Here is a how it descibed in POS2
canInvadeOnlyFrom values: is a colon dilimited list of the transporting units which this unit can perform amphibious assaults from.
A unit which can not perform amphibious assaults may instead disembark during non-combat
can be set to "all", or "none", or a list of some transports. if this option is not present, it defaults to "all"
This should be used more. Landing on a defended, hostile coast requires specialized landing craft, while troops can be moved to friendly ports on almost any ship type. It is also realistic to prohibit tanks from invading, forcing the beachhead to be secured with a infantry & air power.