The case where a man with a low Ikai got a job of a higher level than his Ikai was called Shu, and that where a man got a job of a lower level than his Ikai was called Ko.
下位の位階の者が官位相当よりも高位の官職に就く場合を守といい、高位の位階の者が官位相当よりも下位の官職に就く場合を行という。