Fine fraction substrates
The growing media has a small particle size which is optimal for use in plant propagation in trays.
Medium fraction substrates
The substrate has some larger particles and maybe used in medium sized pots. The larger particles provide some structure to the growing media needed for the medium sized pots.
Coarse fraction substrates
The mix has some much bigger particles for use in big pots and containers. The larger containers require a coarse substrate with the very big particles which provide the necessary structure for the mix.