REQIF12 — Allowed source and target elements on SpecRelationType level

Currently, any SpecObject can be associated with any other SpecObject by using a SpecRealtion. Even though this is very generic and flexible, it is prone to erros, since the user is not restricted by the type of a SpecObject that can be used as source or target element for SpecRelation. So, subsequent tasks have always to ensure that only allowed SpecObject can be linked with each other. Unfortunately, it is not possible to deposit the knowledge about what SpecObjects are allowed in the model itself.

This situation can easily mitigated by adding a sourceType and targetType association to SpecRelationType. Further more, a constraint on SpecRelation must be defined that ensures that the only SpecObjects are linked as source and/opr target that adhere to the SpecRelationType source and target.

Take the issue out of scope, as it is a breaking change

Implementing this issue would introduce a change to the ReqIF XML schema.
This change to the XML schema would make ReqIF files that are valid against
the new XML schema incompatible with previous ReqIF files, thus breaking
interoperability with almost all tools that are on the market now (including
tools by IBM, PTC, NoMagic and many others).

As this is considered a major change, it should be taken out of scope of a RTF.