Split or merge subtypes in physical generation

Sorry if this is a very elementary question.  How do indicate in the logical model that I want subtypes to be merged into a single table on the physical model.  By default they are split into separate tables, and nothing I do seems to change it. I have done it before, but cannot figure it out now. Did it disappear between release 18 and 18.3?  The help documentation doesn't explain it.