As some others have stated above, 24/7 has to do with the duration/amount of time that one is under the dominion of another and PE or power exchange has to do with the amount of control exerted while one is under the forementioned dominion.

Now TPE or total power exchange might sugest in most cases or imply that since the power exchange is in "total" that the time factor is also 24 hours a day 7 days a week.

Alltough that is not in fact the case in every relationship.

Some people are in TPE yet not in 24/7 and or vice versa or some combination of the two and not allways clearly defined.

Hence two different terms to describe time involved and level of power exchanged exist and are not allways mutally inclussive even if it seems that way to casual observation.