Some species (Human, Cyborg, Zabrak, Twi'lek) have different appearance options when you create them, depending on the faction or class you choose. (different implants, skin colour, tattoos, etc.)
For most of them, all the faction- or class-specific appearance options become globally available once you purchase the species unlock or reach level 50 with a character of that species.
EXCEPT for humans.
This only affects the scars and tattoos specifically, which have different (or no) options available depending on your class, but it still seems like an oversight that could be easily rectified, since humans have a level 50 legacy unlock like the other species do.
Also, being able to exactly replicate a character's appearance with a different class can be important for roleplayers and the fact that it's not always possible is... pretty frustrating, actually. ^^