Question on org-agenda-custom-commands tags-todo match syntax

[Niall Dooley <dooleyn@gmail.com>]

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Firstly, I've searched online and even posted to both the emacs and orgmode
subreddits but to no avail.

Consider the following snippet:

(setq org-agenda-custom-commands
'(("p" "List Non-done projects"
tags-todo "-CANCELLED+PROJECT/!+TODO")))

My understanding is that in the tags-todo match that entries to the left of
the "/" are the tags, those to the right are the todo keywords and that you
use a "+" to indicate a condition should be satisfied and a "-" when it
should
not be satisfied. Is this correct?

So in the above snippet the search matches headlines with the tag :PROJECT:
and not :CANCELLED: and a todo keyword of TODO. With this I have the
following
questions which I would appreciate if someone could answer.

Is the "+" on the right hand side before the TODO strictly necessary? I've
seen snippets which have it and others which do not. Perhaps it is only
necessary between multiple todo keywords on the right hand side?

What is the significance of the "!" on the right hand side? Should it always
be present? Again, I've seen snippets with it present and others with it
absent though it is present more often than not. Would it ever appear on
the left
hand side?

Thank you.

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