When a dominant and submissiverelationship becomes sexual. This puts a much higher responsibility on the dom, as the sub usually falls harder into subspace during a scene (sex).
For example, one person can sub to another, but it is only scening while they are having sex.
When two people are in a relationship but not officially dating. It could be because of unwillingness to conform to the Boyfriend/Girlfriend label.
Person 1: "So I just saw Selna Gomez and that dude from Twilight having lunch together"
Person 2: "Oh I didn't know they were dating?"
Person 1: "Well there's no press release, so I guess they're just scening."