A flag is a °marker that occurs on a °nominal and gives information about its °semantic role or °syntactic function.
The term "flag" is a recent innovation in typological linguistics to refer to case forms and adpositions jointly (see Haspelmath 2019). In the past, it was often necessary to refer to "case-marking and adpositional marking", but "flagging" is much more convenient.
flag, flagging (a.k.a. case marker) (STR) = a strategy for encoding the relation in major propositional acts (modifier–referent, predicate–argument), in which there is a third morpheme that encodes the semantic relation between the two concepts, where the dependent concept (modifier in a modification construction, argument in a clause) is an object concept. Flags subsume adpositions and case affixes. Examples: in the plate on the table, on is a flag, and in I dug the hole with a shovel, with is a flag; both are adpositions. (Sections 4.3, 6.2.2)