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These are scripts that users run on IRC, that advertise files, then send them to a user who requests one. Each file is given a pack number like #1, #2, #25 etc. and a user can request a file using either the /ctcp command or /msg command. For example..
/msg botname xdcc send #5
/ctcp botname xdcc send #5
Some channels/networks only allow the /ctcp command sas they see it as being a little safer than the /msg command. You can also request the list of files from a bot using a command like this....
/msg botname xdcc list
However, a lot of trading channels dont allow you to get a list of the files, they would prefer u wait in the channel for the public advertising of the list instead.