Deltamatic and the Delta OSS mainframe system applications run on IBM TPF operating system and applications were written in IBM assembler language and also a language called Sabretalk. In the past 15 years or so, IBM provided C language support, so the mainframe applications are a mix. The external systems like SNAPP access the mainframe "core" using different methods... some structured data and some other special types of service calls. The original external apps would use terminal emulation but most of that is gone now. DLTERM is just a old terminal emulator that could be used to log in and access the cryptic keyboard interface.