There will be a couple of new orders involving landing parasites on carriers in v5.30. At the moment the you can choose the 'Land on Mothership' order when the destination is a fleet. The fighter or FAC will move to the fleet and then land on whichever ship is its currently assigned mothership. For clarification, the description of that order will change to 'Land on Assigned Mothership'.
The first new order is 'Land on Specified Mothership (No Assign)', which can only be chosen when the destination is a fleet. This pops up the extra order box (the one used for ground units or teams) where you can select a specific mothership within the destination fleet on which to land, rather than the assigned mothership. This doesn't change the mothership assignment though, so you can send a fighter to land on a different carrier, perhaps for refuelling, but still use the 'Land on Assigned Mothership' order when it returns home again.
The second new order is 'Land on Specified Mothership (+ Assign)'. This works in the same way as above except that in addition to landing on a mothership, the parasite's permanent assignment is also set to that same ship. This creates an easy way to transfer fighters or FACs between carriers, or to send newly built fighters to their first carrier.
Steve