Loop Material means hook-and-loop/velcro. The loop is the fuzzy soft stuff. The loop material is for attaching a duty belt or other stiff belt. One of those things where if you have to ask what it is, or what it's for, you likely don't need it.
The D-ring is that rigger's d-ring on the right side of the picture that you would hook a carabiner or rope to. If you don't need it, omit it. It's more of a nuisance than anything.
Nylon is more durable and weather resistant than leather over time. Leather tends to be stiffer, and obviously has better aesthetics.