Personal Allowance deduction from income tax

Please correct me if I am wrong. In F6 (TX-UK) questions, when allocating the Personal Allowance, we do not always deduct the full Personal Allowance from savings income because part of the savings income may already be taxed at 0%. Instead, we allocate the remaining Personal Allowance to dividend income to reduce the overall tax liability. I am currently studying both F6 and ATX. I do not remember studying this rule in F6 notes of open tution or lectures, although I have seen it mentioned in ATX. Is this concept actually part of the F6 syllabus, or is it something that is covered in more detail in ATX and should only be followed in ATX paper?