explain system mailbox

[noave]

on https://manual.uberspace.de/mail-spam/#background there is warned

Spam filtering and sorting does not work with the system mailbox. Create user mailboxes instead!

but background of system mailbox and workaround (put |maildrop .spamfolder in .qmail) is missing

[noave]

explain system mailbox background on https://manual.uberspace.de/mail-mailboxes/#system-mailbox