For most installations, pipe flashings can be pulled down from the top of the pipe, but in some situations this is not possible because of obstructions such as cowls, collars, brackets etc. In these circumstances the unique design of the AD series pipe flashings mean that they can simply be cut down the seam, wrapped around the penetration and then crimped together using the VSRFC retrofit clips and fixed to the roof material in the normal manner.
Please see the pipe flashing description for details of the quantity of clips required for each flashing.
Expensive, and how many do you need? You won't necessarily need the number of clips specified for the flashing. For example, V-Seal AD EB 10-90 Flashing says 5 are needed, but for standard drain pipe diameter there will be only enough room for 3 clips after you've cut the flashing to the suitable size. But they seem to work fine.