This means that you will not be able to translate, view, operate on, or audition Kontakt 6 NKI files within any third-party product such as Chicken System's Translator, Kontakt Assistant, or Constructor. You can only access/load/use them in Kontakt 6 - nothing else.
The Kontakt 6 extension will be the same - .nki/.nkm/.nkb. If you try to access those files in Translator, or any Chicken Systems product, you will get an error message.
Specifically, Native Instruments is no longer providing the Code Library (see below) they have provided us with to access NKI/NKM files since Kontakt 4.2. SInce version 4.2, Kontakt files have been been encrypted with 128-bit protection so they can not be reverse-engineered.
So... if you are interested in using Kontakt for authoring, or place some value in taking your information into some other format - such as EXS24, or SFZ, or your keyboard workstation (Motif, Kronos, Kurzweil, etc.), we advise keeping an older version of Kontakt around for creating accessible files. And if you use Kontakt 6, do not resave those nki's. Best Case: use a version v4.1 or less, giving you complete access. With v4.2-v5.8, you can access much of the information, but not modulators, filters, or effects.
Or choose not to upgrade to Kontakt 6. The majority of improvements are geared toward developers, not users, according to many experts who have seen the initial versions.
We believe this is a mistake on NI's part. This does not serve their user base well. Not all will care about this restriction - but we know, firsthand, that many will. We have strongly advocated to NI on the behalf of all Kontakt users, for the last eight years, about the importance of full access to ALL data, but those efforts have ultimately failed. Perhaps NI will reconsider their new position in the future.
Note: Out of courtesy to Native Instruments, we have temporarily removed the detailed-explanation that we previously posted, in anticipation of speaking with them more - Mon Sept 28 2018 - about this. Please be patient.