I have some custom CRS (cCRS). Now, I'd like to work on my QGis-Projects at another PC and want to move my cCRS there.

According to the QGIS manual the cCRS are stored in the "user database". But I cannot find any further information about filename or path of the "user database" in the manual.
I also tried to search for my cCRS-names in the Explorer (search for files including content). I searched in the (typical) user and programm directories, but haven't succeeded yet.

Anybody an idea for me?