Anyone can develop and test Yaffs within your organisation with no restrictions. That's the Yaffs Try Before You Buy feature. We can do this because Yaffs is available under two Licenses, GPLv2 and Commercial.
If you develop and release software which links to Yaffs you won't encounter any Licensing difficulties so long as you meet your GPL obligations, which include releasing your OS and application code. And that code also must be released with a GPLv2-compatible License.
So you cannot take someone else's code and release it under GPL; it's not yours and you cannot change its licensing.
Further, if a customer of yours builds on your software perhaps with another Operating System, or no OS, and releases it then anyone can take any of that code and use it as they wish under GPL provided that they also meet the GPL obligations on releasing OS and application code.
There are people who do not wish to, or are not able to, release their OS and Source code, and they need a Licence for Yaffs.
There are people who expect to use Yaffs with software from other sources, although that software is not licensed in a way that is compatible with GPLv2.
Amongst these, for examples, are eCoscentric and Wind River.
We have an agreement with eCosCentric about your joint use of their code, which they can tell you about.
You cannot release code of VxWorks from Wind River, as the VxWorks License forbids that, and so Yaffs users also using VxWorks need to have a License for Yaffs too.
If you have that, then we can provide you with auxiliary non-VxWorks code to facilitate integrating them.