quickfix hidden marks

In the quickfix documentation there is the following:

If you insert or delete lines, mostly the correct error location is still
found because hidden marks are used. Sometimes, when the mark has been
deleted for some reason, the message "line changed" is shown to warn you
that
the error location may not be correct. If you quit Vim and start again the
marks are lost and the error locations may not be correct anymore.

What are the "hidden marks" mentioned above?
Do they use some character (a-zA-Z) or do they use
some internal capability (c code) that is not exposed in the
Vim script language?

Thanks

Richard
--
Quis custodiet ipsos custodes

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