Q:

3rd texture coordinate in Wavefront *.OBJ file

What is the meaning of 3rd component in texture coords in Wavefront *.OBJ file ? 
File looks like
vt 0.8490 0.1109 -0.2498
vt 0.8580 0.1235 -0.2498
vt 0.8695 0.1338 -0.2498
vt 0.8828 0.1416 -0.2498
vt 0.9135 0.1474 -0.2360
vt 0.8981 0.1457 -0.2360
vt 0.8835 0.1409 -0.2360 

UPDATE
Model was created in 3DS Max and source texture was PNG file.  So texture is 2D

UPDATE 2
When I flip second coordinate then texture looks properly when I render it with OpenGL. 
3rd coordinate 
u1 = u
v1 = 1 - v
w - is not used at all. 

A:

3rd coordinate should be ignored.  Only first 2 are used to map texture.