Typically whether the can is a 2-port or a 3-port solution (or a 2-can solution), plus whether there are additional check valves in the lines.What's the difference between stage 1, 2 and 3?
Tappin' with my G3
On our cars, the line connecting the engine block to the intake manifold has a PCV, to prevent boost from pressurizing the engine cavity. The line connecting the valve cover to the inlet tube has no such check valve; it's really just a filtered air source for when the intake manifold is under vacuum and is sucking out gasses from within the engine cavity.
Different catch-can manufacturers use (slightly) different meanings for Stages 1, 2 and 3, depending on their offerings.
Most of the fluids get collected in the line between the engine block (PCV) and the intake manifold. Probably your Steeda air/oil separator is mounted up top and doesn't catch much -- unless you've got some ring issues or a stuck PCV valve?
Hope that helps