Input variables are used at the inputs of function blocks. You declare VAR_INPUT variables between the VAR_INPUT and END_VAR keywords in the declaration part of POUs.
When called, only the value is
passed (pass-by-value).
The variable can be extended with an attribute keyword (CONSTANT, RETAIN, or PERSISTENT).
Example
VAR_INPUT
iIn1 : INT; (* 1st input variable *)
END_VAR