It means that you can have a 'first choice' and a 'second choice' for the image used to represent each ship size. There is a 'primary' and 'secondary' bitmap entry for each ship class now. So, if you make a custom mod with different ship sizes, and a special shipset to match, your mod can still work with the standard races -- without having to create new bitmaps for them -- if you assign standard sizes as 'secondary' choices on your new ship types.
As far as I know, once the game chooses to use a given bitmap you're stuck with it in both system and combat screens.
[ 12 December 2001: Message edited by: Baron Munchausen ]