The best practice whitepaper currently describes when to choose optional or mandatory. If a value is optional but to be set by a client, it is recomended to always have a variable but define a special value (e.g. -1) if not available.

This is no perfect solution. During review the question for a better solution occurred:
Using sentinel values to provide optional and variable meanings should not be a best practice

To be discussed in the future if we can find a better mechanism.