I think it is sacraments they are allowed to administer- there are seven sacraments -lay people can only administer one i.e. baptism. Bishops have a few extra cofirmation etc.
Yes, crosses on tombs much more interesting answer !
But have seen quite a few tombs belonging to bishops, and don't remember seeing seven crosses on them.
Would be interested if anyone has any references.
Take the point about Confirmation in exceptional circumstances, but it is ordinarily performed by bishops.