When it comes to building the type of physique I like -- powerful looking, lean, muscular and hard like a brick wall -- the three most important training zones to spend a lot of time in are:
1RM/95-100%: This builds limit strength and increases muscle hardness via an increase in myogenic tone and specific hypertrophy of the fast-twitch (more superficial) fibers. Also increases your capacity to recruit the growth-prone fast-twitch fibers.
3RM/90-95%: This is the zone where you can do a lot of mechanical work without killing the nervous system and while still getting the strength-building effect of maximal lifting. There is a HUGE difference on the nervous system between 90-95% and 100%. So that zone allows you to take advantage of the performance improvement from the 1RM zone while getting a lot of mechanical loading.
6RM/80-85%: This is the high load zone where you create more hypertrophy via a high metabolic demand. The longer time under heavy tension (vs. the 1 and 3RM zones) activate different growth processes/pathways than the heavier loading zones do. It's a training zone that allows you to stimulate muscle growth at little nervous system expenses, a way to continue building muscle while allowing the nervous system to recover.
The original layer workouts used all three zones in the same workout. But recently I've been playing with a microcycle layering instead of a intra-session layering, rotating through the intensity zones in various workouts BUT also using different methods for the same training zone within one session. So you have a layer effect during the session and also during the training cycle itself.