Aasimar, tieflings, and genasi are staple characters for almost every Planescape campaign. However, they are classified as "native" outsiders, which doesn't make any sense if your planetouched character isn't native to the Material Plane. Some people choose to reclassify these characters as humanoids instead. That, however, diminishes their power level, as being an outsider makes you immune to charm person and the like. It also changes your options for such spells as alter self and polymorph. (For example, a tiefling cannot use alter self to become a human, but he could use it to become a flamebrother salamander. No one who is not already an outsider can use polymorph to become one.) So, if you want your planetouched characters to be outsiders, what should you call them if they aren't "native"?
Simple: Change the "native" subtype to the "mortal" subtype. This is really what separates planetouched characters from full-blooded outsiders. A planetar or gelugon cannot die of old age, but it can't be raised or resurrected either. A planetouched character can do both, yet still has an undeniable connection to the planes in his blood.
Some, however, might object that making planetouched characters immune to charm person is too powerful for a planar campaign. According to the Planar Handbook, planetouched make up 20% of the population of Sigil, and that's a lot of people who your spells can't affect. One solution for this could be the use of spell keys that allowed humanoid-affecting spells to affect mortal outsiders. However, that belongs to another article.
Why: Just because it makes sense.Pro: It allows planetouched to be different from regular outsiders without referencing the Material Plane.
Con: Some people would rather have planetouched as humanoids instead.
The PLH gives an incredibly high elf and halfling percentage, which makes absolutely no sense. The bariaur are herded (heh) into the "other" category, the exemplars as well, the 'zerai are not even mentioned, and all the various planetouched are thrown in together (a 20% total might be OK, but tieflings are way more numerous than, say, earth genasi).
Oh, and the listed population is only 250,000. I'm nost sure this is in accord with 2E info.
I chose to ignore the Sigil stat block in the PLH.