Indicator variables can be used with main variables intended for I/O, although certain indicator variables cannot be used for input (for example, those appearing in a WHERE clause).
PP2 uses indicator variables to handle nulls sent to or returned from the database or truncation of a character or byte string when placed into the corresponding main variable.
Indicator variables are documented in Teradata Vantage™ - SQL Stored Procedures and Embedded SQL, B035-1148.