Richard: A sovent system is a special drain/vent system within a multistory building which eliminates the need for a separate waste (soil) and vent stack. Basically a wet vent. The main "oddball" componets are deaerator and aerator fittings--you need (at least) one of these components at each floor.
Pedarrin2: I don't have any experience designing one of these, but I imagine from an installation standpoint it's no more difficult than traditional waste/vent plumbing work. The fittings I mentioned above come with connections for the stack inlet, outlet, branch and waste inlets...anything more special than that is built into the components (probably just baffles). I don't think all jurisdictions allow this alternative, so that would be the first step...