I'll get a picture of the bolts.
Engineered fill could mean one of two things.
Sometimes the term is used when a certain bearing density must be reached - they bring in fill and compact it until it reaches that bearing density. I'd typically state something like "Backfill Structural Fill areas in uniform lifts of 8 inches maximum in uncompacted thickness and compact to not less than 95 percent of maximum dry density, following ASTM D698, at moisture content within 2 percent optimum for material."
The second use is "flowable" fill. Basically low grade concrete. The advantage of it is that it is essentially self compacting. Typically you limit it to 10' lifts, so if you have a deep but relatively small volume it can be a lot cheaper to just fill it with concrete rather than do an endless series of 8" lifts.
I will be backfilling and compacting inside the structure before placing the floor so that I don't get any settlement.