move 123 to numeric-item, this moves the value +123 to your numeric field allowing comparisons such as
if numeric-field = 123...etc
move "123" to numeric-item, this is moving a literal (i.e a string) to your nueric-item field. This should result in an error as you can only move numeric values to numeric fields.
High values will differ depending on your platform, on an ebcidic platform (mainframes & midrange such as AS400), all characters are represented by a two character value. e.g the values 1 , 2, 3 etc are actually represented as F1, F2 and F3 in ebcidic. In EBCDIC, high values is the maximum value a single chracter can contain and is represented by FF, low values is represented by 00. It actually relates to hexadecimal (base 16 numbers where the values 0 - 15 are represented by 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E and F) which most of these machines use for storage (as opposed to binary on a PC).
Usually high values is good if you are finished reading a keyed file, and you wish to manually force an EOF. You simply move high values to the key and do another read, this should put you automatically at the EOF point. Similarly for low values, if you move low values to your key field before a read, you know that you will be at the very first record in the file.