You can find "Split" tool under "Modify" tab.
|Common use with "Split" tool|
"Split" tool can apply to wall or when sketching a floor/ceiling/roof/stair during sketch mode. Also, you can use split in family editor environment while creating the geometry during sketch (e.g. extrusion).
What many people don't know is you can use split on wall in vertical plane like in 3D view or elevation.
How about families? I use "Split" on beam (structural framing) family quite often, too.
In addition, you can split a Line-Based family with this tool.
As for detail items, you can use "split" tool on Line-based detail items as well as repeating details, too!
Hope you will find this little trick useful.