I went through this too. first need to know what gun it's for. Probably a picatinny, but they make ones specific for TRG etc also.
Then ring diameter
Then height. For AR15 and DTA (straight line) your probably want High. Ultra high is due out soon.
Then taper. They make flat, 44 MOA, and several other tapers.
Their product numbering system actually has something that corresponds roughly to these specs, but I forget what order and their exact code and I can't find a link right now.
They're great mounts, you won't be disappointed. From now on whenever I mount on a rail I'm gonna use these mounts.