The symbols are short forms of the CAT operators.
|| is *CAT (Concatenate without altering the spaces between the two values)
|> is *BCAT (Concatenate with a blanks space between the two values)
<| is *TCAT (Concatenate and remove spaces between the two values)
The symbols are not recommended. Because they consist of characters that may be variant, they can appear differently on different workstations or printers. They can also be incorrectly translated when source members are copied from one system to another.
I tend to replace them wherever I find them with the mnemonic forms.