When it applies.
Part 4 rules apply only while racing unless a rule says otherwise.
PFDs become compulsory in three ways: flag Y displayed afloat with one sound before or with the warning signal (for that race), flag Y displayed ashore with one sound (all day afloat), or a class rule, notice of race or sailing instruction that says so. Wetsuits and drysuits do not count. You may take it off briefly to change or adjust clothing. Outside those cases rule 1.2 leaves the decision to you.
Changed in 2025
Class rules, the notice of race and sailing instructions can now switch on rule 40.1 directly.