As it reads you’re allowed to be brought on and off furlough as many times as your employer sees fit. The only stipulation when it come to your employer claiming is that you’re furloughed for at least 3 weeks at a time.
So in theory you could be furloughed for 3 weeks, work 1 day, furlough for another 3 weeks.
Outside of that your employer can also ask you to come into work for various reasons that don’t directly generate revenue for the company such as training, meetings etc. So in my company people have been rostered to attend OPC’s, LPC’s, Wet drills etc.
A side point is that you can also work for another employer whilst on furlough as long as your furloughed employer authorises it. So at my company people have been bumping up their furlough working for Tesco, Asda, NHS etc. Some of our moonlighting furloughed FO’s are currently earning more per week than some of our captains who are still flying for the company.