The NLS_LENGTH_SEMANTICS environment variable specifies the default length unit for Forms items and PL/SQL variables of character datatype which do not specify an explicit length unit. Specifically, it applies to Forms items whose datatype is CHAR, ALPHA, or LONG, and whose Data Length Semantics property is null, rather than BYTE or CHAR, and PL/SQL variables of type CHAR and VARCHAR2 whose declaration does not specify an explicit unit of BYTE or CHAR.
If the NLS_LENGTH_SEMANTICS environment variable is set to a value of CHAR when the form is compiled, then such items and variables will use character length semantics; otherwise, they will use byte semantics. Note that if this environment variable is changed, then all PL/SQL program units in relevant forms should be explicitly compiled (through Compile All).