Question: What's the difference between Re/Im and Re~/Im~?

Where Re(M) seems to work fine, Im(M) does not
I already found out which procedures to use instead: Re~(M) and Im~(M).
But still, it looks like

Re(M)=Re~(M)

am I right, or what's the difference between those two?
BTW, I didn't find a page about Re~/Im~ and was wondering..

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