[2.1] Opinion Enum in 10.18 for Opinion SDO? Why not a Vocab?

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/175

See the follow up comments from community members in the issue for  
further context


[2.1] Opinion Enum in 10.18 for Opinion SDO? Why not a Vocab?

in 10.18 there is a Opinion Enum: Why is this defined as a enum? Where  
pretty much everything else in the spec is a open or closed vocab.