Yeah- a grave reason to me would be imminent death or the absolute unavailability of sacramental confession. But I think sometimes leeway is given.
For example I believe that canon law says that you must confess in an enclosed confessional unless there’s a grave reason otherwise and I’ve had many confessions outside of the confessional.
Either way if I am in mortal sin I stay in the pew even if I was invited to Communion in this way by a priest.
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