I saw it in a question once and want to know what it is.
Light Clay raises the duration of Reflect and Light Screen from 5 turns to 8 turns.
Light Clay extends the duration of Reflect and Light Screen to 8 turns instead of 5.
Light clay raises the duration of light screen or reflect or both, from 5 turns all the way to 8. It isn't really nessecary IMO but you can use it if you'd like.