They are different families of code and can all have releases at some point. I am most familiar with the ERS 5000 units and their code an will use it to demonstrate, but it follows the exact same idea.
ERS 5000 6.2.x is their "classic" code and the most stable. Just bug fix releases at this point.
ERS 5000 6.3.x is their "advanced" code with new features and options, but not as time tested as 6.2.x. Make sure you know what your doing before loading this on production equipment. Expect bugs to show up as they refine this code family.
ERS 5000 6.6.x is their "experimental" code that involves complete rewrites of the code. It unlocks exciting new abilities on supported equipment, but leaves backwards compatibility behind. For loading on lab gear only as it is so new. No one in their right mind would use this on production gear. Interesting to watch though to see what they come up with.
So just because you have a release with a higher number doesn't automatically make it the best code to use. Many companies have decided to stay with the classic code which is the most stable, so they will only care about 6.2.x releases. I use classic code on the user edge and advanced code in the core, so I look at releases from two code families.
