[help] how to use message_attach

Couldn’t understand what "int low ,int high " of message_attach’s parameters
mean? From the help:
int low, int high:the range of messages that you’re interested in.
But what’s the range of messages? How to get the range of messages or when
will this range be satisfied? I copy the example program of message_attach
and comment all sentence of message_attach ,then I get the same result. I’m
really confused .
Could anyone help me to illustrate the usage of message_attach? Thanks a