[BCF 2.1] Questions regarding extension.xsd documentation

Hello everyone,

In the documentation release_2_1, it is not explicitly stated whether extension.xsd is mandatory or optional, and seems to be left to interpretation.
On the other hand, in 'BCF file structure' chapter, it is clearly stated that project.bcfp is optional and bcf.version is mandatory.

@GeorgDangl's suggestion in issue #151 is to adopt the following recommendation:
"If there are extensions in the BCF file you're reading, treat it as the allowed values. If there's no extensions, all values are allowed."

To me, it implies that extension.xsd is indeed optional.
Unfortunately, I have not been able to find any other clearer suggestion of what or how extension.xsd should be used.

Hence the following two questions:

What is the necessity regarding extension.xsd?
Has a common recommended pratice been chosen and adopted by implementors regarding usage of extension.xsd?

Best Regards,
Guillaume
a new BCF implementor