The VAxx family are variables which are local to the script running. There are 100 of them, named from VA00 through to VA99. When a script begins running, they are initially all 0 (integer type), but can be assigned any value.
Because they are local, they cannot be used to share information about state between the scripts of an agent, or between agents. Commands like MESG WRIT and CALL, which allow execution to pass to another script, do not preserve local variables. GSUB, because execution stays inside the script, does. CAOS is a special case: it may or may not preserve VAxx variables depending on an argument.
Playing with some variables (first numbers, then strings):
SETV va00 5 OUTV va00 5 ADDV va00 10.5 OUTV va00 15.500000 SETS va01 "Hello, world!" OUTS va01 Hello, world! SETS va02 "\"" ADDS va02 va01 ADDS va02 "\" said Jamie." OUTS va02 "Hello world!" said Jamie.