According to WMO 1.2.3.2:
A code table is a list of choices where only one of the choices can be selected at a given time. The selection is accomplished by coding the number of the desired entry in the appropriate place in the GRIB2 Section. A flag table, on the other hand, is a list of choices where any combination of the choices can be selected at a given time. The combination is accomplished by turning bits of a bit string on or off. Code and Flag tables are also categorised by the Section number in which the code or flag table is referred to.
...