We use APP at the moment (until it goes out of fashion...)
Basically, for each child we have a sheet with 2 levels on it; their current level and the next level. Most of my class (year 3) are somewhere within levels 2 or 3.
The sheet is divided into statements that show evidence of level 2, and statements that show evidence of level 3. Every term, we use a different coloured highlighter to shade in any statements we feel the child has met.
This sheet is then passed onto the next teacher, who will know the child's weaknesses and strengths. It also shows when the child made the progress (by the highlighter colour).
As a year 3 teacher, the sub levels thing is tricky. Children are expected to make 2 sublevels' progress each academic year, apart from in years 3 and 4 where they made 3 sublevels, ie a whole level, within the two years. This means that in either year 3 or 4, a normal child is likely to only make one sublevel progress.
Not all schools use APP, and different places use it in different ways. We do it for all 30 children, for maths, reading and writing. Lots of highlighting.
It's also very much up to the teacher's perception of how many bits need to be highlighted to declare a child a certain sublevel. I was taught that a 2b, for example, would be the majority of level 2 statements, and a 2a is all of the level 2 with a few of level 3. A 3c is a few more.... 