Yes vague!

The FAA only says that it can't block another passenger's access to the aisle. Airlines can add to, but not contradict, FAA regs so many simply put in place a blanket rule saying no car seat between a passenger and the aisle.

I've been contacted by a couple of airline people saying that their companies allow "low profile" car seats, specific brands and models, to be placed on the aisle because they decided that these forward-facing seats do NOT block anyone's access to the aisle.

I'd love to confirm this but I'm not sure how. I believe, not sure, that Delta is one of these companies with a more detailed policy.

I could do a call-out on one of the private airline FB groups but that wouldn't be official, get someone who now works for Delta to look it up. Better yet, any of you get into conversations with any crew on your flights, any airline, feel free to ask and report back here.

Unfortunately, the contents of the flight manuals are not made public. Possibly the reservations agents are up-to-date but unlikely. They usually don't know all the in's and out's of car seat installments and might just advise booking the window seat, to keep it simple.