Hello,
The high and low value of a byte are the highest and lowest possible value the byte may contain. X'00" is the lowest value and x'FF' is the highest value you will use on the mainframe. Other systems with a different collating sequence might have somthing else.
There is a bit more here (scroll down):
http://publibz.boulder.ibm.com/cgi-bin/ ... lr10/1.2.7?