"Some" and "mainly" and on the second question, not all of them, you could gain marks elsewhere.
Anyway, you think the paper is inaccessible, I think it's basic, let's agree to disagree.
So you cannot explain your terms "limited ability" or "lower ability". OK. Well, the human brain is generally wired to have almost unlimited ability to learn and absorb knowledge, including abstract concepts like letters, numbers and their infinite combinations. This is human nature.
If someone displays "limited ability" in your terms, there are usually one or more causes to it:
a) they have a cognitive dysfunction or developmental disorder meaning their brain functions in an unusual way. ASD, dyslexia, sensory-related conditions. Then we're talking about SEN.
b) they have a somatic illness or physiological dysfunction or affective medical condition not directly related to the brain and nervous system but affecting cognitive functioning. Prolonged illness or adhd that may be a) or b) or depression etc. Then we're talking about SEN and/or often medication.
c) they have a genetic disorder directly affecting cognition. Down's syndrome etc. Then we're talking about SEN.
d) they have never been taught properly and haven't developed foundational skills and as a result cannot develop more complex and advanced skills. Then we are talking about the failure of educational system.
This is a crude picture but just to give an idea.
So what exactly are you talking about when you say "limited ability"? SEN or educational failure? Or are you just saying "they're lower ability" as in "they're born stupid"?
No, I'm not a teacher, that's probably why I don't brand anyone as being "limited ability".