RN3 has it right, LCUs are withdrawn automatically. not sure on this, but if an LCU is loaded on ships, it can't be withdrawn & you'll pay a PP price.
some LCUs have the note 'out of theater', their components aren't added back to your device pools when they withdraw.
for withdrawing airgroups, always remove any reserve aircraft before allowing them to withdraw. some w/d airgroups will be scheduled to return later, others have the 'withdraw' option grayed out and can only be disbanded. if the airgroup must disband, empty its pilots to the reserve pool, & consider downgrading its planes to last-line aircraft before disbanding the group.
for ships, fly off any CV or FP air components to the local base before withdrawing. these airgroups can be useful trainers, & you won't take any PP heat if the ship is withdrawn w/o its planes.