So here is a part where i draw a line at some point and use that line to do a boolean difference to cut it in half for a parting line:



The result is the part split into 2 peices:



I also can use a method of running a plane through the area I want to part and choosing the Isect command to create intersection curves of the area, which i can use to break the geometry in different ways:



If you want to get into these further, post a sample and we can look at it.