What it is
An .nki is a Kontakt instrument. It holds the zones, the groups they belong to, the envelope and filter settings, and any KSP scripting the instrument runs.
The format is proprietary and undocumented. There is no published specification, the layout changes between major Kontakt versions, and Native Instruments has never shipped a writer for anyone else to use.
Instrument Name.nki Samples/ Piano C4.wav Piano C5.wav
What is inside
A Kontakt instrument either points at a folder of samples or carries them inside itself. The second kind is called a monolith, and the extensions around the .nki tell you which parts of a library you are looking at.
.nki one instrument .nkm a multi: several instruments loaded together .nkx the sample container in a packed library .nkc the cache file that speeds up loading
Vocabulary
Kontakt says Instrument where an Akai MPC says keygroup program and Ableton says multisample. They are the same object, a set of samples mapped across a keyboard.
Inside an instrument, a zone is one sample on one key range and a group is a set of zones that share settings. Logic calls a zone a zone as well, SFZ calls it a region.
What reads it
Kontakt, and Kontakt Player for libraries licensed to run in it. Nothing else opens one with any completeness, which follows from the format being closed rather than difficult.
Kontakt reads its own older instrument versions as well as the current one. That backward compatibility is the whole reason a converter can produce a usable .nki at all.
What Sample Miner writes
The Kontakt option runs the instrument through ConvertWithMoss, which writes an older .nki instrument version. Current Kontakt opens it and the mapping plays.
It is not a modern Kontakt instrument. The export dialog labels the option "Kontakt (older .nki version)" rather than plain Kontakt, because the difference is one you will notice the moment you look for anything beyond the key mapping.
The limits
A converted instrument carries the mapping and little else. Key ranges, velocity layers, root notes and loop points come through. Scripting, custom interfaces and group level modulation are what separate a commercial Kontakt library from a set of mapped samples, and none of it survives.
An instrument saved by a newer Kontakt does not open in an older one. Libraries move forward through versions and never backward, which is why the Kontakt version is part of a library requirement list rather than a footnote.