Patterns in manual: difference between EXPR and USER?

[Vincent Lefevre <vincent@xxxxxxxxxx>]

In the manual:

~c USER         messages carbon-copied to USER
~C EXPR         message is either to: or cc: EXPR
~e EXPR         message which contains EXPR in the ``Sender'' field
~f USER         messages originating from USER
~L EXPR         message is either originated or received by EXPR
~t USER         messages addressed to USER

Why do some patterns contain EXPR and others USER?

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Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.org/>
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Work: CR INRIA - computer arithmetic / SPACES project at LORIA