The reversal rate is the percentage of pending commissions that have been reversed (see commission reversal) for a given time period.
Reveral rates vary according to merchants and verticals. Thus, reversal rates are logically higher for shoe merchants as many customers return the shoes.
Reversal rates can be taken into account by affiliates for choosing a merchant, but they are only shown in some network interfaces. However, reversal effects are indeed included in EPCs.
Too high reversal rate can be sometimes a hint for merchant fraud.