Nominal/coupon interest is the rate attached to the bond itself and is applied to the nominal value of the bond. This is a fixed rate of interest on the bond.
Effective rate is the rate calculated taking into account the substance of the bond (i.e. being issued at a discount and redeemed at a premium). The effective rate is essentially the market rate and is the original effective rate.
Any change to the effective rate due to any impairment is then the revised effective rate.