It means the unit type which they are. The list for Revolutions is this:
0 = Infantry
1 = Cavalry
2 = Dragoons
3 = Irregulars
4 = PreDreadnought (PDN)
5 = Advanced BB (BB1)
6 = Modern BB (BB2)
7 = Battlecruiser (BC1)
8 = Advanced BC (BC2)
9 = Advanced Cruiser (CA1)
10 = Modern Cruiser (CA2)
11 = Protected Cruiser (PC)
12 = Light Cruiser (CL)
13 = Experimental CV (CV1)
14 = Converted Hulk CV (CV2)
15 = Purpose Built CV (CV3)
16 = Dreadnought (DN)
17 = Monitor
18 = Ironclad
19 = Man O'War
20 = Frigate
21 = Commerce Raider
22 = Submarines
23 = Steamer Convoys
24 = Clipper Convoys
The first section is the country tag, followed by this unit type number, and then the name of that individual unit.
Also, if you are into ships, you might be interested in one of Paradox's latest ventures,
East India Company. I'm not sure if it is this era of ships and naval warfare which you are interested in, though the game is looking rather fantastic!
