What is 1.5 pair cable?

I've come across a spec for 1.5 pair cable and wondered if it's just three conductor cable. It would be nice to have this confirmed, as well as have an explanation for why it's called in such a roundabout manner. Is the word pair a specific terminology in this context which somehow conveys something "(un)shielded 3-conductor cable" does not?

• Link? It may be 1 twisted pair and a loose conductor, which would have some uses. "Pair" does sort of imply twisted to improve balance. Dec 1 '20 at 20:12
• I think it is just a special case of a multipair cable. So in n-pair you substitute n with 1.5. The difference with 3-conductor cable would be that in 1.5-pair, you can have a twisted pair bundled separately from the additional conductor. Dec 1 '20 at 20:14