Thank youYes. Controlled impedance.
If you have a single conductor, then to have an impedance, you need a reference plane.
that could be a copper plane, or free space.
When you have two conductors, as in a differential pair, then you have two impedance, one between the pair, the differential impedance, and one to the reference plane, as above,
A differential pair in free space, the dominant impedance is the differential impedance, as there is little reference to a plane,
on a PCB, then there are both single ended and differential impedances on the pair, if the two tracks are far apart, but clos to a ref plane, then they impedance might be constrained by the single ended impedance.
For instance, things like ethernet after the transformer , or differential microphone inputs typically are differential , with no plane as reference.
I'd say, if your asking, don't stray from the "standard" traces over a solid reference plane,
https://en.wikipedia.org/wiki/Stripline
https://en.wikipedia.org/wiki/Microstrip
by Jake Hertz