The relative probability of an intervening cloud locating at low/high redshift (further away from us). This probability is parametrized with a power law, P(z)=zβ, where z is the redshift. As this index increases, the absorption lines are concentrated toward to the right, indicating that there are more absorptions due to regions that are far away.

This relative probability of an intervening cloud having low/high density. The probability is parametrized with a power law, P(N)=N-α, where N is the column density. Given a fixed total column density, more low density clouds (by increasing this index), instead of few high density clouds, will create more absorptions. High density clouds can be saturated and, in turn, waste the column density budget.