x1 is a limb/appendage/extremity of body/entity x2 used for purpose x3
Any extremity used for manipulation, support, mobility, flight/swimming, etc. Usually, it can move and be controlled actively by x2 (if conscious). The term may or may not be applicable to heads, tails, mandibles, antennae, or plant parts, but occasionally may. It is more general than tuple or birka. It is also useful for talking about limbs which are rather anatomically different from human limbs: fish fins or tails, spider legs, snail feet (possibly), elephant trunks, etc. In winged insects/entities (including machinary), it can refer to wings as well as legs (especially in birds as opposed to insects, but even then as well); care must be taken if one wishes to be more clear. "Tentacle": tu'urtubra. See also torso, xadji.