There was an assortment of passes handed out over the years. I think some at the NW/DL merger. Then an IT meltdown in ATL that caused a massive disruption for all employees. Some were given out for Covid efforts. Some were given to pilots who took the early retirement.

Initially they had expiration dates and precluded best available (1st class) seating. Many (all) of those that were awarded became non-expiry and available for use in retirement(Technically they expire when you expire...no right of survival). Lots of rules but you can give them away to peeps on your pass rider list. Sometimes you can give them away to anybody but they must be accompanied by you or someone on your pass rider list. I think pilots that have been here 10is years might have gotten 3 tranches of 2 tickets per event. A few more if you took the early retirement package.

That's the "big picture" that is prolly 80% accurate.