/copy members are a way to include other source members in a RPG program.
The syntax for a /copy is:
/COPY [[<library>/]<source file>,]<member>
If the library isn't specified, then the library list will be searched.
If the source file isn't specified, then the source file the program is being compiled from will be used.
For example (in the copy member):
/IF NOT DEFINED <somevariable> /DEFINE <somevariable> // code goes here /ENDIF
Thus, if the member is copied in more than once, the code that is in the member will only be actually included once.