[2.1] missing defined open vocab for Pattern Types?

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/173

in section 4.6 there is the pattern_type property.

Its type is open-vocab. But there is only the vocab defined in the  
properties description rather than a vocab from the section 2.1/Stix  
Vocabs.

Is this on purpose?