[Freudian]

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From a programming point of view, I would say it's likelier that it's the number of connected sessions to their servers than the number of occupied seats.

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From a marketing point of view, I would say it is much more likely to be the one that gives the highest player count.

I am pretty certain that one seat occupied=one player. And if you look at how the player counts increased dramatically in a very short timespan during the 3 000 000 000th hand promotion it seems likely that the increase was due to people adding tables with 10 minutes to go instead of 8000 people just happened to log on en masse right when the hands were about to be played.