Could you please explain what does this mean exactly - as this shows many times in the instruction f.e. “Slow Firing weapons can only be
equipped by single-model units” - does it mean that i can’t have two NCR Heavy Troopers with Minigun in squad in Battle Mode? Same goes for PA for example ; I would really appreciate the answer 
Well, in Battle Mode, you should use only single-model units, so every single unit card is just for one model to avoid the sharing of Limited Use items (p.32) with any model on the Battlefield or for a better “record” of the game.
If you play normal mode, or settlement, it means the same, you could equip these weapons/drugs/perks/heroic/leader/power armor cards on a unit card with a single model, so if you want to use 3 Settlers, one with minigun, another one with psycho chem, and the last one heroic with double-barrel shotgun, you need to separate all these models with 3 different unit cards.
You can have two “single-model units” that are the same unit type. The nomenclature is a little bit confusing, but each “unit” consists of one or more models. So you could have two units with one model, or one unit with two models. However some items like slow-firing weapons, items with diminishing effects, etc, can’t be used by units with more than one model because you can’t track the effects on multiple units simultaneously.
It’s also worth noting that you can’t have two Miniguns in a Battle Mode force unless you have 750+ caps. Heavy weapons are capped to 1 in 500 caps battles, 2 in 750, and +1 for each additional 250 caps.
Hope that helps!
HI @boyleer,
This has been passed on to a Dev @Ethan. Hopefully they will be able to answer your question.
Modiphius-NathanP
This is the correct response.