Wavefront obj file, Faces have 3 texture coordinates

Wavefront obj file, Faces have 3 texture coordinates

I exported a .obj from blender to be loaded into a C++ program i'm writing
Everything is fine but I was wondering when Parsing the faces I noticed
that there was an unused texture coordinate on the third point in the face
data.
f 1/1 2/2 3/3
//f 1/s 2/t 3/? Its vt3 that is the coordinate in question
What is this used for? and should I be keeping it? Thanks for any insight!