The ULM645 makes it possible to use Mamiya 645 ProTL Lenses in combination with the Ultima 35.
Mamiya 645 Pro lenses can be set manually to the desired aperture setting in the contrary of Mamiya 645 AFd lenses.
The ULM645 can only be used in combination with Canon bayonet based SLR bodies and will not work with Nikon bayonet based SLR bodies.
The use of a Mamiya lens relates to your sensor size as the same focal length in 35mm format, so f.i. a 45mm Mamiya lens will have the same perspective on a 1Ds as a 45mm Canon lens;
for the smaller sensors the maginification factor will apply of course.