Files with zel extension are used by the Zel computer language used for creating MIDI data.
ZEL file extension - Zel MIDI source file
What is zel file? How to open zel files?
File type specification:
The zel file extension is associated with with Zel, a computer language for creating MIDI data, designed to augment existing MIDI sequencing tools.
When a .zel file (which describes operations and how to generate MIDI data) is run through the Zel language interpreter, it produces MIDI data which can be loaded into a standard sequencer program. From there it can be played or edited using all the standard features of a MIDI sequencer.