vCenter Upgrade versions

Hi,

We are running vCenter Server 5.1.0 and all of our ESXi are running the same versión. We want to start the upgrade to versión 5.5.0. Maybe it is nonsense but we do not know the difference between 5.5.0Ua1, 5.5.0Ua, 5.5.0Ub and 5.5.0c.

Is it 5.5.0c the latest versión(and the best option)? or is it better go for 5..5.0Ua1? (Does exist any difference?).

I tried to find info but I did not find any clear answer.

Thanks in advance,

Ricard.