What's this extra piece that came with my Nautilus?

[mommymaci]

Hello,
I recently purchased a Nautilus from BRU and inside the plastic packaging with the manual, there is a piece that I don't know what it's for? Black webbed strap with red plastic. It is probably something so obvious but I don't know what it is and can't find it in the manual.
And I was also wondering if it is a LATCHable booster? Or do I just put it on the seat w/o LATCH when in booster mode? I am currently using it with the harness.
TIA,
-Catherine

[o_mom]

Check the section on backless booster mode. My first thought is that it is the shoulder belt guide strap for backless mode. Usually a long strap with a plastic oval-like ring on the end.

ETA: p 51-54 of the manual shows its use