This was discussed recently but I cannot find the thread.
The question is concerning Dreams and Nightmares. Are they the stuff of Mentem or Imaginem? Or both?

The reason I ask is that some characters are in the Magic Realm in a Boundary where the stuff of Dreams and Nightmares are made manifest. The debate is whether this should be a Mentem or Imaginem Boundary, or could it be either.

In Mysteryes revised there are Mysteryes to work wth thase part of a dream, and those part is a Imaginem subject, more real and more "msytical". But the Dream capacity herself is a Mentem construct, and you can affect or create dreams in a sleeper or dreamer without any Mystery with mentem spells out of them.
You should see what aspect is more concernent with the story theme and work with that.

Mentem from outside the dream, Imaginem from inside, according to TMRE, dream magic.

The way I see it is that Dreams are Imaginem, but they are naturally produced/affected by the mind, so that mind-affecting magic (Mentem) can indirectly affect dreams by influencing the mind that influences them.