Appearance defines what an object looks like in the game, with various appearances defined for creatures, placeables, and doors. Some game mechanics rely on a creature's appearance rather than race. (For example, certain non-fleshy appearances cannot be petrified by flesh to stone.)
A part-based appearance is a special appearance for creatures where the definition of what the creature looks like is divided into pieces, each individually modifiable. While changing the appearance of a part will change what is seen on the screen, this does not change the creature's designated appearance. Furthermore, the options for each part are dependent on the appearance designation, so part-based appearances are not interchangeable. (For example, when creating a human player character, the part-based "human" appearance is used. Various parts, such as the head, can be changed, but the appearance is still designated "human". Furthermore, the choices are not the same for a different part-based appearance, such as the part-based "dwarf" appearance.) That is, this is a case where there is a disparity between an appearance as defined by the game and what appears on the screen.