what is a "Work PFD" vs "This is NOT a Work PFD"?

IIRC I saw an inflatable vest stamped "Work" and a solid foam normal vest stamped "Not a Work PFD".

https://www.grainger.com/category/safety/water-safety/life-jackets-and-pfds

don't see any such definitions here. Will post pics of "Not a Work" later.
Don't see any official info on "work" vs not work. Is "work" anything official or just more durable fabric. Hard to imagine makers stamping otherwise legal vests "not work" unless forced.

 

It is the requirements for the buoyancy, see 46 CFR 160.001-2 and 46 CFR 160.053. Generally a "work" PFD must supply at least 17.5 lbs of buoyancy continuously while being completely submerged for 24 hours. A "standard" TYPE II or III PFD needs to only supply 15.5 lbs of buoyancy, and you really can't work in a TYPE I PFD (supplies 22 lbs). "Work" PFD's also generally are TYPE III PFD's, not TYPE I or TYPE V hybrids (i.e. a "work" PFD generally will not turn an unconscious person face up as OSHA requires personnel oversight and a rescue skiff to be present).

Edit: Here is a typical modern TYPE V work vest we used to use for our overwater riggers.

Stearns 3000003509 | Fisheries Supply https://www.fisheriessupply.com/stearns-inflatable-work-1471-vest-automatic/3000003509

Edit 2: This is also a work vest we used for deck personnel.

https://www.archwaymarinelighting.com/product/stearns-work-vests-10/