LOW-VALUES and HGH-VALUES are Figurative constants in COBOL.
LOW-VALUES refers the lowest value in collating sequence.
(Hexadecimal Value – x’00’) – Minimum possible value for a character (EBCDIC)
HGH-VALUES refers the highest value in collating sequence
(Hexadecimal Value – X’FF’) – Maximum possible value for a character (EBCDIC)
Collating Sequence is the sequence of a system which decides the equivalent numeric value for each character typed in keyboard and the precedence b/w characters.
In PC, ASCII is the default collating sequence, while in Mainframe EBCDIC is the default.
So usually in Mainframe code, along with checking if a field has SPACES or ZEROS, field is also checked to see if it has these values (LOW-VALUES and HGH-VALUES) and then initialize accordingly.
Collating sequence of ASCII and EDCDIC can be found in below link
You can also set the required collating sequence inside your COBOL program.
Below post clearly explains it.