This article is from the MachTen & CodeBuilder FAQ, by Jaime Julca firstname.lastname@example.org with numerous contributions by others.
If you start a MacTCP application (such as Eudora, Gopher, or Newswatcher)
while MachTen is running, that application will automatically run using
Tenon's TCP stack. If you attempt to run that application without MachTen
running, the application will simply default to using MacTCP.
When you install MachTen, the MacTCP configuration parameters will
automatically appear in the MachTen Networking Control Panel, so you don't
have to re-enter your Internet addressing information. It is not necessary
to deinstall MacTCP, since it will very compatibly coexist with MachTen.