It's a Brooklyn Hipster version of a Montreal deli.
So, like all Brooklyn Hipster restaurants, the ingredients are all sourced from "sustainable" "heritage" sources, and everything than can be made in-house is.
And, since it's a Montreal-style deli, they serve "smoked meat" instead of pastrami. This is sort of a quarter of the way between pastrami and corned beef (I think if it seems like it's more rather than less spicy than pastrami, that's because of the house-made mustard, which is very hot).
The sandwhiches are NOT piled high -- which to me is a GOOD thing, as I think most traditional delis almost disgustingly overserve. OTOH, the sandwiches are almost 50% cheaper than at Katz's.
The BIG problem is that because their approach is so "artisanal", they can't make enough product to keep up with demand. So they run out of meat by about 3 PM every day.