Golly, it was a long time ago.
The Level Descriptions are part of the National Curriculum, certainly amended by Dearing in 95 and maybe again later? Help please Nanaej or anyone else who remembers! A lot of people were involved, teachers, advisers and inspectors. I was involved in writing some of the stuff for ICT level descriptions. I am not sure why it wasn't based on norms, but I think it was an aspirational target. Here are Levels 5 and 6 which are the expectation for 14 year olds.
"Level 5
Pupils' writing is varied and interesting, conveying meaning clearly in a range of forms for different readers, using a more formal style where appropriate. Vocabulary choices are imaginative and words are used precisely. Simple and complex sentences are organised into paragraphs. Words with complex regular patterns are usually spelt correctly. A range of punctuation, including commas, apostrophes and inverted commas, is usually used accurately. Handwriting is joined, clear and fluent and, where appropriate, is adapted to a range of tasks.
Level 6
Pupils' writing often engages and sustains the reader's interest, showing some adaptation of style and register to different forms, including using an impersonal style where appropriate. Pupils use a range of sentence structures and varied vocabulary to create effects. Spelling is generally accurate, including that of irregular words. Handwriting is neat and legible. A range of punctuation is usually used correctly to clarify meaning, and ideas are organised into paragraphs."
You can see why I get cross when people say that pupils don't understand different registers or genres!