The glass temperature transition is for glass, polymers, etc. (amorphous or semicrystalline materials), but not for leather.

The glass transition is a second order phase transition. For materials that have a glass transition temperature, below it they are hard and brittle and above it they are softer and "rubbery".

It doesn't have

Tm is temperature of melting. Tg is glass transition temperature.

The -125 degrees indicate the transition temperature of Polyethylene from glass.