Let's be honest the seatbelt sign can go on for a variety of degrees of turbulence varying based on route, airline, captain. I've had it on for periods where I feel certain someone has forgotten it, I've also had it on while tray tables went flying and my friend was puking into a bag (his one and only case of air sickness in maybe 200 flights).
There isn't a degree marker on it but I'd assume - I could be wrong - that there are times it's turned on and drink service can continue and there are times the FAs are warned to sit down and stop as it could be significant.
That said, FA could also, on occasion, use it as an excuse. Which ties into J being more flexible perhaps.