I've not heard that pixie dusting is a different process. Well, in my experience, for example, people sign up for FE groups. Sometimes there are so many people that the whole group breaks out into smaller groups (so everyone doesn't have to give massive amounts of gifts). Some people in one of the groups may decide to go ahead and gift everyone (all groups) - that's pixie dusting. Or someone who gives gifts that isn't even on the FE list - that's pixie dusting.
There are really no set rules for FE groups. Each group sets up whatever "rules" they want - how many gifts; price; delivery days; whatever. Many groups just say "do what you want".