There are 2 special parts for mobile build areas. Build area parts will display while building but will be invisible when launched. In campaign these build areas parts can be unlocked from blueprints at the buy shacks.

The drydock version of the mobile build area. It can be placed on a vessel or a structure and it creates a drydock that can be accessed from the map similar to structures.
Note: The arrow indicates the build/launch direction.

The runway version of the mobile build area. It can be placed on a vessel or a structure and it creates a runway that can be accessed from the map similar to structures.

Note: You can rename the build area that appears on the map by renaming the part.

Parent vessel: The vessel you have the build area on.

• Your parent vessel must have enough fuel to fill the new vessel.
  
• Your new vessel must be smaller than the parent vessel

In campaign, you must have passengers seated on the parent vessel to crew the new vessel.

In sandbox there is no requirement to have Nautikins on board a vessel that is deploying a new vessel with a build area, they will magically teleport to the new vessel. Let's just pretend they swam over really quick. It is more fun that way. When recalled they will swim home. In campaign you must have spare Nautikins in passenger seats on the vessel in order to allocate crew members. When recalled they will be seated back on the vessel or dropped wherever they were if no seats are available. There is a misc. setting to turn on the requirement in sandbox mode (maybe you prefer no teleporting), however this setting is only available at certain times (when not in build mode) to prevent problems with crew allocation. This setting is not available in campaign mode.

When distance return to build area is on, you will always be returned to the build area nearest your vessel that matches the vessel type (aircraft vs ship). If you disable this option you will be returned to the last build area your vessel launched from or Portus if that is not available. This may be useful for building bases to attack from in multiplayer or something.

Build area vessels can only be launched from vessels that are larger than them. This does not apply to structures / bases which do not move and will more or less be treated like Portus and ignore the size requirement. For some examples: You cannot launch a jet from another jet of equal or smaller size. You cannot launch an aircraft carrier from a jet. You can launch a jet from an aircraft carrier. You can launch a scout ship from a cargo ship. You can launch a gyro from a jet. etc., etc.

Build area parts are renamable and will show the name on the map.

If the build area is destroyed while you are using it you will be sent back to Portus and any unsaved changes to the vessel you are working on will be lost (risk of using mobile build areas). It could save some sort of temp vessel later but there are several reasons why I do not want to do that atm (Timing issues, needs to be destroyed fast, multiplayer, performance, added risk is sort of desirable, etc.).

In campaign, vessels can be refunded/recovered at mobile build areas.