Don't know why there is any confusion when Dave 'Diddy' Splane explained it so masterfully??
Seriously, thanks to the above posters for the detailed explanations - most helpful, as was this overview.
https://jwfacts.com/watchtower/generation.php
A word of caution, just don't ask a current PIMI JW to explain t to you. I've tried. They haven't got a fu$$en clue, and quickly gravitate to accusing you of lacking faith or being an 'apostate'!