Again, if you hover your cursor over the tool you want explained, then press "F1" for the "relative" help file topic [may need to refine it to "Split Draft" from the links menu on the right].....the trick to split drafting, is to setup a "reference plane", or a "surface" to drive the split from definition.
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If you're asking about why you'd use these functions instead of how, Split Draft is used to create draft from an existing parting line on a part such that the edges used to create the draft remain stationary, and the face tilts back from that edge. If the edge splits a face, you can draft the faces on opposite sides of the edge in opposite directions. This is the most common (95% in my experience) usage of draft when the mold splits a face of a part.
The Step Draft is something that is only used in special situations, and I don't know them all. We used it in rotomolds frequently. It creates a ledge, or mismatch when the parting line has a step in it. It is used to keep a face smooth even if the parting line on the face has a step in it. Split draft will make a single face faceted if the PL is stepped, but there is no mismatch between the PL from one side of the mold to the other.
If you're not sure, leave step draft alone. It's more complicated, and can produce unexpected results in the molded part. Talk to your molder/mold designer if you're not sure.