The difference between total depth and working depth in a gear tooth is

Pitch

Clearance

Addendum

Dedendum

is uniform

varies parabolically

varies linearly

is zero

uniformly varying load

constant point load

constant distributed load

uniformly distributed load

to change the structure of steel

to increase the surface hardness

to increase resistance to heat and corrosion

all the above

Tempering

Carburising

Annealing

Nitriding