Between the key words VAR_IN_OUT and END_VAR, all variables are declared that serve as input and output variables for a POU.
NOTICE
With this variable, the value of the transferred variable is changed ("transferred as a pointer", Call-by-Reference). That means that the input value for such variables cannot be a constant. For this reason, even the VAR_IN_OUT variables of a function block can not be read or written directly from outside via <functionblockinstance><in/outputvariable>.
Example:
VAR_IN_OUT iInOut1:INT; (* 1. Inputoutputvariable *) END_VAR