Mario's maximum X speeds while flying. The first 8 bytes are facing right, the second 8 are facing left.
Each of these sets are split into four groups of two bytes each (for maximum left and right speeds). Only the first two groups are actually used, for flying without X/Y and flying with X/Y respectively.
Also, in case it's not clear, the maximum speeds in the direction opposite of Mario's are from pressing that direction with B to slow down (e.g. <+B when facing right).