v2 spec UNKNOWN geometry type

Nobody has claimed this yet.

The spec says about the UNKNOWN geometry type: "The specification purposefully leaves an unknown geometry type as an option. This geometry type encodes experimental geometry types that an encoder MAY choose to implement. Decoders MAY ignore any features of this geometry type."

What is the purpose for this wording? Do we have a use case for this? Are encoders really going to use that value for their experiments? Why only one value? If we really need values reserved for experimentation I suggest we define, say, all enum values above 256 as experimental or something like it. It is often useful to keep the zero value as some kind of _unspecified_ value that should never appear in the data, but implementations can use the value internally as a default after initialization of some datastructure or to signify a deleted feature or so.

Read the v2 specification section defining the UNKNOWN geometry type and review the four comments for any existing decision. Clarify the intended use of the value, whether experimental values need reservation, and the meaning of zero; done means the project has an agreed rationale and corresponding specification wording.

Written by the indexing model from the issue text.