The distinction is rule based, that's all. It's like "Animal" and "Vermin". Vermin are clearly tiny animals, but in D&D they want to make a distinction because they want certian spells or weapons to affect animals by type.
I don't see that in Conan. An Outsider is any creature from outside the normal plane of existance. An Aberation is a creature that has been altered or would otherwise seem normal but for some physical...well...aberation.
Conan RPG, however, doesn't really make a distinction between Animal and Vermin for the purposes of targeting. In D&D, a giant spider is still a spider and so is classified as Vermin. In Conan, this is not the case. An aberation can be a demon, trapped in this plane or merely called a demon by locals or something.
What you have to look at for effects is and whether they work on a particular creature or not is in the title line. Conan RPG doesn't pay as much attention to creture type as D&D, and overrides much of what D&D is based on. In Conan, a creature's weaknesses are listed in it's profile rather than by type, and any vulnerability that is exploited or created by a spell or poison or other substance (like silver) is give with it's entry. Creatures just don't ahve the succeptibility that they do in D&D, so you can't base rule inferences on D&D.
Does that make sense?