I think on primary school reports (in our area) they tend to state that level 2 for end of KS1 and level 4 for end of KS2 is on target, i.e. average, where they are expected to be.
So if you have that information, and you note that your child's actual levels are above or below that ,then surely it is self-explanatory?
The sub levels ,c,b,a (in that order) don't mean much more than "beginning to", "working towards" and "working solidly at".
So, basically, if you have a level 2 or level 4 at the end of those Key Stages, don't worry!!
If your child is above those levels, be proud of them for working hard.
The main thing to remember is...it is not a competition! Children can suddenly pick up a couple of sub-levels ,or sometimes they will not move for a bit. The sub levels do not denote intelligence!
Someone will probably now tell me that is not the official language, but that is my understanding of how it works. 