It is hard to be completely definitive about what counts as cover, different players will have access to all sorts of different terrain so common sense is the best guide. The main criteria is that cover has to be reasonably substantial. Basically, it is up to the players to define what constitutes cover in their games, but here's a guide to some of what we designate as cover.
Crates, Rocks, Woods, Jungles, Ruins, Buildings and Vehicles
Cover which is notably more or less substantial than these can have a different saving throw value if you choose. For example, really light cover such as high grass or a chain link fence that conceals troopers rather than protects them gives a cover save of 6+. Fortified bunkers, trenches or heavily reinforced emplacements will give troops within them a cover saving throw of 4+.