I have flat ones for my regular and embroidery machines.
The round ones were for one of my sergers. I have used one of the round ones in my regular sewing machine in a pinch, but you have to make sure, that you place it exactly in the correct spot. Not sure if I would recommend it, but sometimes you have to do strange things. I live over one hour from the nearest store and was in a bind. Nothing happened to the machine, but now I make sure that I have enough flat ones. p.s. I still have the round ones, even I no longer use this particular serger. You never know when you need them.