Tempering is what gives chocolate its gloss and clean snap. Pick your chocolate and how much you are working with, and this gives you the three temperatures and the seeding amounts.
Why the three temperatures matter
Cocoa butter can set in six different crystal forms, and only one of them (form V) gives a glossy surface and a sharp snap. Melting to stage 1 wipes out every existing crystal. Cooling to stage 2 lets new crystals form, including the unstable ones. Rewarming to stage 3 melts the unstable crystals and leaves form V to spread through the chocolate as it sets.
White and milk chocolate temper at lower temperatures than dark because milk fat interferes with cocoa butter crystals and softens the chocolate.