Said by Michael Pemulis to to Miles Penn during a heated game of Eschaton. This is no doubt a reference to the Polish-American scientist and philosopher Alfred Korzybski, whose dictum "the map is not the territory" is a key principle in neuro-linguistic programming.
The Eschaton game highlights the distinction between the two, as the lines become blurred due to the falling of snow, which the game's elaborate rules do not account for. Some players fail to understand that the snow, which falls on the map, doesn't fall on the territory that the map represents, and therefore should not be considered a factor in the game. However, one player throws a missile/tennis ball at another player, rather than an area on the map, prompting the following outburst from Pemulis, the E.T.A's foremost Eschaton authority:
Players themselves can't be valid targets. Players aren't inside the goddamn game. Players are part of the apparatus of the game. They're part of the map. It's snowing on the players but not on the territory.... You can only launch against the territory. Not against the map. It's like the one ground-rule boundary that keeps Eschaton from degenerating into chaos. Eschaton, gentlemen, is about logic and axiom and mathematical probity and discipline and verity and order. You do not get points for hitting anybody real. Only the gear that maps what's real.
On the subject of maps, Wallace coins the term 'to eliminate one's map' as a euphemism for committing suicide. References to people 'eliminating their map' feature throughout the novel.