Autocast abilities; Default Active ability

Is it possible, with the normal World Editor, no programs or anything, to make certain abilities autocast that normally aren't (without triggers... I mean just the spell). I thought you couldn't at first but then I see default active ability, and apparently it sets autocast abilities to on right when unit is made if the ability has autocast. However, it seems to have default active ability options for like, purge, and other stuff...
Can I make such spells autocast? and if not, what is the point of Default Active Ability?

Someone else can answer the autocast thing better than I can (it involves dummy spells etc). An ability under defaul active abilities, If im not mistaken means that the abiltity ignores its required tresearch dependancies. I think

Default Active Ability means that the unit is given that abilities "turnon" order at the moment it is created, for most abilities the "turnon" order is null, but that doesn't remove them from the list.