BrepFace.Split only splits a single face in a Brep. It does not split the entire Brep into one or more Brep pieces. For an example, run the
The Rhino SDK does not have a function that splits a Brep with a cutting curve. To do this, you’ll will need to make a surface from the curve and then use Brep.Split.
If you have user text on the target Brep, then you might consider harvesting it before performing the splitting. Sometimes, user data attached to geometry does not survive traumatic operations such as splitting. In these cases it is better to attach user data to an object’s attributes (instead).