ETTR, ETTL or both?

Exposure to the Left or Right. It's kind of a "Zone System for digital photographers". The expression refers to where the data are shown in the histogram in your camera. It's about under - or over exposing your images.
Som people prefere one before the other. I think both are usefull. EETL is udsefull for avoiding burned out highlights. EETR is useful for scenes with important white things - like snow or a wedding dress.