I’ve brought an example a long time ago regarding Brawler somewhere in how absurd damage resistance can be, especially if you bring it up to high values.
In Horde you’ll have max 1000 Health and max 500 Stim unless buffed via cards or perks or something. As explained previously by someone else, Stim is like a shield that has to break first before damage is applied to your health (few exceptions) so it’s something like a “second life” of a way and especially noticeable on Anchor, Pilot and Combat Medic since those regain Stim very quickly, two of which can spread it to teammates as well.
When it comes to damage resistance, it usually to both Health and Stim unless specifically specified like for Infiltrator, hence why it is usually the preferred choice if you had to choose between the three.
Just beware that damage resistance doesn’t scale linearly, meaning if e.g. you had 75% damage resistance damage done to you is 1/4 of what they usually do, with 90% it would be 1/10 obviously etc. You can build some seriously tanky classes if you put the combinations together properly. Combat Medic and Blademaster are good examples for that. Both can basically be invulnerable for a period of time (not really but the health regen makes it feel like being invulnerable at times).