The instructions mention making the application thicker by using a "proper aggregate." What would be considered a proper aggregate?

This is what is needed: Up to 5" (12 cm) thickness may be poured with the addition of aggregate [well-graded, washed, dry pea gravel 1⁄8" (3 mm) or larger]. First mix as instructed. During placement add equal parts of the aggregate to mixed self-leveler by volume, mix until completely coated. To ensure proper bond, all aggregate and substrate must be completely coated with the underlayment mixture. Work fast since you will not have a long pot life. Keep in mind you might need to top off with the product mixed without the aggregate if a smooth surface is needed, typically required if vinyl floorcovering is being installed. If this is needed you will need to re-prime.