Yes, confusing, indeed.

I've always taken "back-to-back" tickets to mean that the next ticket (itinerary) starts immediately following and from the same airport as the previous ticket. That is, you complete the flights on the first ticket, then immediately start the next one.

"Nested" has always meant to me that in the middle of one ticket, you start and fly another, regardless of whether entirely or partially.

There, have I confused everyone even more?