Different kinds of transports have different ways for units
to get on and off. For instance,
ships can dock, or use their boats to enable land units to get on and off.
The tables ferry-on-entry
and ferry-on-departure
specify how much terrain units will have to cross on their own.
[example]
Observe that enter/leave costs can be used to make one-way trips. For instance, paratroops jumping out of a plane should be able to leave cheaply, but have an entry cost so high that they can only reboard in a later turn.