As TheGruffalo says sub levels don't technically exist only levels so it isn't possible to list what a child needs to know/be able to do in order to achieve c or b, a is easy as it is everything in that level descriptor.
Isn't that semantics though because each level has a breakdown of criteria (set out in the APP grids) and if you know how many of the criteria a child meets, you can determine a child's sub-level:- low = 'c', secure = 'b', high = 'a', they are just different terms for the same thing aren't they?
Where exactly does it say, in any of the official guidance, that a child must have achieved every criteria to be working at sub-level 'a'?
To quote from the Assessing Pupils Progress Handbook (my bolding):
Tick the box for Low, Secure, or High as follows:
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Low – meets at least three of the criteria for the level across some of the AFs
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Secure – meets most of the criteria for the level across the full range of AFs
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High – meets all, or almost all, of the criteria for the level across the 4 AFs with some highlighting of criteria in the level above.
Common sense suggests that many pupils get to grips with some criteria from the level above before they are fully competent at every criteria from the level below.
It's also very interesting to read this, from Getting to Grips with Assessing Pupils' Progress:
Highlighting the criteria pupils have met across a range of evidence provides a visual picture of their strengths as well as gaps in their learning. It’s also a useful basis for a conversation with pupils and their parents or carers.
Well yes, it would be incredibly useful if teachers shared this level of detail with parents. Look, here's the grid showing all the criteria for level 2. As you can see dear Jonny has demonstrated a sound understanding of x, y and z. At this point criteria a, b and c are the ones he needs to focus on to move from being at a low level 2 to a secure level 2 or a 'c' to a 'b'.