For a panel designed for use with an MPPT CC or GTI the nominal voltage is Vmp, as stated. And the nominal current would be the nominal wattage divided by Vmp, which is equal to Imp.
For panels intended for use with as PWM CC to charge batteries, the "nominal" voltage of the panel is sometimes taken to be the nominal voltage of the battery bank it is to be used with. Using that convention, a panel with Vmp=18 may be called a "12 volt panel." Similarly, a panel with Vmp=36 may be called a "24 volt panel".
In that case, the nominal current through a PWM controller will be wattage divided by Vmp and not voltage divided by 12v.
When writing down, calculating with, or referring to the nominal voltage of a panel, you need to make it clear which of the two conventions you are following.
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