thanks for the feedback Kristian. IMO, "I want to be able to log in using Twitter credentials" sounds like it answers the question "What do I want to do with the system so I can achieve my Business Goal?" and is, therefore, a Capability.
However if the Business Goal is "Make system login easier so that I can attract more users", then you can say that the Capability is "able to log in using Social Account credentials" and "login with Twitter" answers the question "How do I deliver this capability?" i.e. a Feature
I find that the following help to distinguish a Capability from a Feature
It's all about putting things in the right context and Impact Mapping is ideal for this.
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