What is a Parser Skeleton?

Hello together,

"Parser Skeleton" is crucial for understanding the Bison Exception
<https://spdx.org/licenses/Bison-exception-2.2.html>, but I didn't find a
proper explanation of that term.

So either:
     Where can I find a proper explanation of "Parser Skeleton"?
Or:
     What is a "Parser Skeleton"?


Thanks in advance and best (of the best) regards,
Jo



PS:

I also started a github discussion
<https://github.com/akimd/bison/discussions/99> with these words and
afterwards recocknized your reference on this mail address in the readme.