This is part of the Norms Based Title system.
The letter/number following ‘N:’ on a crosstable shows what norm performance level was achieved at that event. In essence, norms are achieved when a player scores more than one point above what a player at that norm level would have been expected to score. (This means, incidentally, that it is mathematically impossible to earn a norm in an event unless you score at least 1.5.)
N:S means senior master performance level (2400)
N:M means master performance level (2200)
N:C means candidate master performance level (2000)
N:1 through N:4 means level 1 through 4 performance level, respectively (1800, 1600, 1400 and 1200)
Norms performance levels are computed independently of the awarding of titles, so someone who has, for example, the Candidate Master title (C) could still be listed at having performed at a Level 1 or lower level in subsequent events as well as at a level higher than the highest norms-based title he or she has earned to date.
Performance levels and titles are only computed after an event has been rerated at least once, so they may take a week or so to show up for an event. Titles at the Candidate Master Level or higher have ratings requirements in addition to the requirement to earn five norms, so those titles may not be earned and awarded until after the ratings requirement has been met.