Certain APIs of the IBM mainframe operating system z/OS (formerly known as MVS), IBM makes you sign an NDA before it gives you the documentation to them.
For example, IBM mainframes have specialised processors called zAAPs. Physically these are just ordinary CPUs, but they've been configured in the firmware to only be allowed to run certain types of code. The reason for this, is IBM licenses the OS on a per-CPU basis, and using zAAPs instead of normal processors to run work results in lower per-CPU licensing costs. Anyway, there is an undocumented API that programs can call to offload work on to zAAPs. And IBM makes vendors sign an NDA before sharing this documentation. (If they didn't, anyone could just run arbitrary code on a zAAP, defeating the purpose of the whole thing.)
z/OS also has an API called FAMS (File Attributes Management Service aka IGWFAMS). You need it to perform certain operations on files (or datasets to use the mainframe terminology.) The IBM manual which documents the FAMS API (called the "DFSMS Advanced Customization Guide") is made available to ISVs under NDA only. (As to why this is under NDA, there has never been a public explanation.)
I myself have never used either of the above APIs, nor have I signed any NDA for them, nor do I know the technical details the NDAs protect. I just know they exist. And I know that quite a few IBM mainframe ISV products depend on one or the other to work.
For example, IBM mainframes have specialised processors called zAAPs. Physically these are just ordinary CPUs, but they've been configured in the firmware to only be allowed to run certain types of code. The reason for this, is IBM licenses the OS on a per-CPU basis, and using zAAPs instead of normal processors to run work results in lower per-CPU licensing costs. Anyway, there is an undocumented API that programs can call to offload work on to zAAPs. And IBM makes vendors sign an NDA before sharing this documentation. (If they didn't, anyone could just run arbitrary code on a zAAP, defeating the purpose of the whole thing.)
z/OS also has an API called FAMS (File Attributes Management Service aka IGWFAMS). You need it to perform certain operations on files (or datasets to use the mainframe terminology.) The IBM manual which documents the FAMS API (called the "DFSMS Advanced Customization Guide") is made available to ISVs under NDA only. (As to why this is under NDA, there has never been a public explanation.)
I myself have never used either of the above APIs, nor have I signed any NDA for them, nor do I know the technical details the NDAs protect. I just know they exist. And I know that quite a few IBM mainframe ISV products depend on one or the other to work.