Let's continue that s3 example. There are many products branded as "s3 compatible", but they all have slightly different definitions of what that compatibility means. Not to mention that the s3 api itself is a moving target, and doesn't even have a version number you can reference, so the best you can do is list which features you do or don't support. Some of these products actually have native APIs which IMO are better than S3, partly because they can learn from mistakes in s3s API.