Flag/Code table 8 -- Scanning mode
from the ECMWF publication "Binary data representation - FM 92 GRIB", May 1992.

Bit      Value    Meaning
1        0        Points scan in +i direction
         1        Points scan in -i direction
2        0        Points scan in -j direction
         1        Points scan in +j direction
3        0        Adjacent points in i direction are consecutive
         1        Adjacent points in j direction are consecutive

Notes:

(1) i direction: west to east along a parallel, or left to right along an 
X-axis.

(2) j direction: south to north along a meridian, or bottom to top along a 
Y-axis.