Hi Vincent. Done mine recently at PDR. Suction method is used in areas where it is difficult or impossible to place the flattening tool (kind of metal rod with a flat end) to push the dented area from the inside.
If the dent is on your side doors, normally they would use the flattening tool, access via sliding the tool into the window panel (from top - of course with masking tape protecting the window from being scratched), and push the dent out from within.
For roof top dents, then suction method will be used. They would normally ask if your ride is on original paint work. Of course they will inform you of the possibility that the paint will peel off, and to inform you of the risks involved. But mine was 99% close to perfect and paint work was not affected at all.
Hope this helps.