Ansys Employee

I can't remember what Regions does!  If you create a Register (under Adaption) then split by Mark. Note, as you're trying to split a mesh you'll not get a perfect shape as only cells entirely in the Register will be added to the new (or kept by the old) surface. 

For precise splits use the Geometry tool(s), the surface splits in Fluent are to help with errors or where less precision is needed.