For golf clubs they actually MEASURE this swingweight! It has to do with the weight of the head of the golf club compared to the rest of it. Or, as a Scotsman is supposed to have said: "It's the heft, Laddie".

One thing about fiberglass is that it is a little heavier than carbon, but the fiberglass rod tend to be shorter so the swing weights might balance out in the end.
https://midcurrent.com/experts/whats-th ... trout-rod/

I don’t know if swing weight is such a factor in the lighter line weight rating rods. I’m usually fishing rods rated for 7-9 weight line and in those, swing weight becomes an issue after a lot of casting. I’ve gone to shorter rods like the 7/8 weight CGR and the G.Loomis Short Stix series. They are both 7’6” and make casting for hours a lot more fun than my old 9’ models.