Overs are placed over your muscle, but under your breast tissue, and unders are placed under the first layer of muscle (called partials really, but surgeons call them unders as it’s under compared to over. Full unders are generally used in reconstructive or gender surgery) the sizes are the different sized implants we all have, and projection (extra high, high, medium, low) is how far they project compared to their base measurement
Hope this helps. Xx