Once upon a time Leister Productions developed the Windows version and the Mac version of Reunion alongside each other. They then decided to sell the Windows version to Sierra who renamed it Generations. Leister Productions concentrated on the Reunion for the Mac from then on.
At first both Reunion and Generations were fully compatible with each other and the .uds family files were interchangable. As time passed by the two programs were developed separately and became less compatible. Now, the only way to transfer data from one to the other is via GEDCOM, which although good , is not perfect. It only takes a few seconds to transfer your data from Generations to Reunion via GEDCOM.