Poker Push/Fold Chart Instructions

I'm wondering how to read the push/fold charts. The one I'm looking at, for example, shows a column for number of players (8 through SB) but it doesn't explain how to count the players. Does that column mean players left to act? Or which seat you are in? Do you have to be first in the pot (vigorish)? What about hands that have callers or raisers before you? Basically, I just don't know what the assumptions are in the chart and it doesn't say anywhere on that site.

Source.