Previous Micro Focus products referred to Mainframe Transaction Option (MTO) as a facility to enable CICS, IMS and JCL applications that have been migrated from the mainframe to be maintained, developed and deployed on Windows or UNIX platforms. This term has now been replaced by Mainframe Subsystem Support (MSS).
You might still see the term MTO referred to within some areas of the product - for example, supplied support files, pathnames and within the documentation. In these instances the terminology has remained the same.