All vehicle attachment have some specification as to what it can be attached to.

For instance Briefing Auditorium attaches to any vehicles with Silhouette ≥ 5 while Electronic Counter-Countermeasures Suite attaches to any vehicles

With the description of a Holo-Tank, we can obviously say that it can only be attached to a big ship...

The only meaningful text within the description is "Holo-tank is a catchall term for a type of holoprojection system used aboard starships and in military facilities."

Every ships that has the Class "Star" and Type "Star" are under the Battleships sections

Does that mean that "Holo-Tank attaches to Battleships only" (beside military facilities which isn't a ship)

Simply put: No. Unless the last part of an attachment's description mentions any restrictions as to on what kind of vehicle an attachment can be installed, you're free to add it to any vehicle of your liking.
Less bluntly and more informatively put: That being said, you might wanna approach this with common sense. Where would you put what basically amounts to a big table with maps and holograms on it? Big warships and command centers, obviously. An X-Wing? Nah, not really. A big command speeder? Absolutely! A speeder bike? Get the frag out of here.
If a player can come up with a reasonable way to put a big table in their vehicle and actually use it once they're done, they should be allowed to put a big table in their vehicle and use it once they're done.

I understand the point of "using common sense". I'm looking more toward something specific .... as I'm preparing text to create new cards....as you may have seen here

Fair enough. Set a hard cap, then. Silhouette 4 or more seems appropriate.

Maybe I could write it like I did for the Enhanced Prisoner Lockup (Any vehicles with enough room to hold prisoners suite (GM's discretion))

Although I'm just wondering if they referred the word starships as to a ship that is of the Star class while writing this.... maybe not....

21 minutes ago, Mefyrx said:

Although I'm just wondering if they referred the word starships as to a ship that is of the Star class while writing this.... maybe not....

You're seriously overthinking this. In this game, "starship" just means it uses Piloting (Space) as opposed to "vehicles" which use Piloting (Planetary).

A starship is a ship that flies between stars. Effectively, that means exactly what HappyDaze said.