MasterGwydion
Emperor Mongoose
Yeah, it gets a bit convoluted. I cannot find the source currently either. PoD Book2 page 16 says this in the Timeline...Harriers being mostly trade police is how it plays out in my head as well, which means it is probably written in there somewhere, but digging around for a primary source I'm not finding it. I don't suppose you can recall the relevant book or passage?
While I feel like while Harriers were used most often against civilian traffic, they were certainly a part of the empire's military strategy. We know from PoD 2 that Sindal arose after "The threat of raiders caused a few worlds to band together and fight the fall of night." and that as the empire rose they conquered planets and drove back raiders, "its focus was always on defence and expansion, instead of consolidation." This says to me that Sindal wasn't just protecting its own assets, they were harassing enemies as well, in the form of worlds they were conquering or raiders they were pushing back.
Harriers certainly saw naval combat. During the civil war yes -- we know, for example, from PoD1 that "Admiral Gani Peras avoided direct engagements where possible, preferring to use her light Harrier and Wyvern class ships to attack enemy supply lines and fuel dumps." -- but in the description of the Rorix Command Vessel as well, where we hear "Naval operations were carried out by squadrons of small ships, often led by a larger vessel. One standard formation comprised three Harrier commerce raiders and a Rorix command vessel, which provided support and carried boarding parties to deal with vessels crippled by or held under the guns of the Harriers." which suggests that the use of Harriers was a common Sindal naval practice.
"884: King Oleb XIV establishes the Drinax tax. Harrier ships are built to enforce payment of this tax." but this isn't exactly definitive.