None!
You can use the Units static class to access any explicitly defined units:
Units.m;
Units.J;
// so on and so forth..
You can apply prefixes to any unit using standard operators for multiplication and division:
var km = Prefixes.k * Units.m // equals Units.km
You can also build complex units with the same rules:
var spectralUnit = (Prefixes.G*Units.yr)^1.5 // equals Gyr**(3/2)
Finally, you are able to parse out FITS-standard units strings into the units system:
var fitsString = "m";
var tokens = FITSUnitTokenizer.Tokenize(fitsString);
var parser = new UnitParser(tokens);
ASTNode<Unit> node = parser.Parse();
var unit = node.Evaluate();