David, a single flare is where you put a conical swivel tip on the forming screw, and just make a mini-funnel on the tube end.  A double flare is where the tool forms a larger single funnel end, then a special tool tip folds the oversize funnel's outer edge back into the first funnel, and compresses the folded metal tight back onto itself, creating a double-thick funnel.  A bubble flare creates a hollow bubble for different brakeline joints.  See video below, along with others to learn about the different types of tube ends.  Each is designed for a different pressure standard or whatever local rules are.