Print/Advanced/Select origin

I'm rather confused by the behaviour of the Print/Advanced dialog. The option to set the "Origin of print area" is not described in help very clearly. I expected the X/Y values to be in milimeters on printed paper (as is true in AutoCAD), but it seems they're not working that way. The "Select origin" option appears to return x/y in drawing units? What is the logic behind this dialog? ThanksVaclav