The way i read it is that if you have a WP from your primary employer then you could also work part-time or in a different capacity for a second employer provided that the type of employment IS NOT on the list of prohibited jobs. I.E. a teacher in a school with a work permit could work for a language school/ work online without needing that employment added to the WP. To take a recent example; if you were a diver working with a WP and you were called to do volunteer rescue in another part of the country you wouldn't need that volunteer position added to the WP. It does not allow a teacher to work part-time as say a waiter/ waitress simply because they have a WP.
This was the last re-entry permit i got. It was the very last page in my passport, I didn't even have room for any other stamps. Never had an issue, told the officer that I was getting a new passport while I was home.