As far as English is concerned, Higher candidates only have two attempts at each NAB. There are two to sit: Close Reading and Textual Analysis. They cover the range and complexity of A - D grades, but they are not graded as such. A candidate either passes or fails, but the absolute minimum to pass a NAB is half marks - a C. NAB units have to be passed to complete the award of Higher English. They do not count towards a mark for the final exam in the sense I think you mean here, but they do count as a completion of the exam and they are detailed on final certificates.
The award is a combination of NABs, folio and exam. The folio is 20%, teh exam CR paper 40 % and the Critical Essays Paper the final 40%.
In addition to this, candidates must submit a folio: two pieces each of 1300 words, consisting of two different types of writing.These pieces will be supervised and the candidate must sign to say they are not plagiaried and that no-one else has helped them. Infringements are taken seriously and if caught out, the candidate will be lucky to get away with just being denied an English Higher.
Re-hashed pieces from their Standard Grade folios will not work. The required sophistication of the writing is far removed from even Credit level SG.
The candidate should be studying techniques for CR and TA as par for the course, which will obviously help with NABs. They should be studying each subject for a minimum of four hours outwith their formal lessons. If they are not, they may be in for a very rude awakening come the prelims!
Hope this covers what you were asking? If not, I'll happily try to clarify!