In SH2/DC it is very easy to create a new vessel/airplane/submarine that wasn't previously in the game. You just pick a 3D model of the game and create a new UDF file (Which is simply a text file with that extension) and give the unit those characteristics you want. For example, you can add to the game a Colorado class US battleship by picking the Texas 3D model and reworking an UDF file to have the proper number of guns, gun size and such. Many mission creators and modders added new units that way.
Now, if all players in a multi game do not have those new units, the game will crash. That happens typichally when trying to play missions created by the community (Not always the readme states clearly that new units were added), and shouldn't happen at all with the stock missions in the standard games.
Hope that helps