The conference paper SS-520 Nano satellite launcher and its flight result SSC18-IX-03 from the 32nd Annual AIAA/USU Conference on Small Satellites linked in this answer describes the use of rhumb-line control in several places.
If I look at Wikipedia's Rhumb line it is a mathematical concept, and I don't see how it would apply to a normal launch to orbit trajectory:
In navigation, a rhumb line, rhumb, or loxodrome is an arc crossing all meridians of longitude at the same angle, that is, a path with constant bearing as measured relative to true or magnetic north.
Question: What exactly is "rhumb-line control" in the context of a launch to orbit trajectory? How does the use of the term reconcile with the definition in Wikipedia?
Images from the linked paper, where is the rhumb line?