Develop one or more vocabularies for RelationshipType/Relationship

[johnwunder]

This field is used to describe the nature of a relationship between STIX components. It currently uses the controlled vocabulary type, however STIX has not defined any vocabularies to use.

Due to the number of places this field will be used it's likely we'll want to develop more than one vocabulary. As we identify these vocabularies it's probably worth creating new issues for them individually.

[johnwunder]

The first step is to identify all types of relationship pairings (Indicator->Indicator, Indicator->TTP, Indicator->COA, etc). We could then solicit input from the community, but this would be a very high level of effort (in particular to get a full set).

We don't expect this to be fully resolved for 1.1 but we can solicit input and potentially even develop first-cut vocabs for a few of them.

[johnwunder]

This issue is being deferred to a later release in keeping with the associated STIX 1.1 Proposal.

[athiasjerome]

Related to #291