In addition to the dat-format ships mentioned by Jeff, there are ships whose cfg file points to the GR2/DAT file of other similar ships. This type of units lack a DAT/GR2 file of their own, and they are known as "proxy copies".
The Patroclus is an example of that; here the relevant strings of her cfg file:
Code:
ClassName=AMCPatroclus
3DModelFileName=data/Sea/NAMC_Rawalpindi/NAMC_Rawalpindi