According to this verse Christ is prophetically pointed out in Psa 110:1-7 as an eternal priest, independent of fleshly descent, a king, and superior in dignity to the Levitical priests.
According to the order (κατὰ τὴν τάξιν)
According to the rank which Melchisedec held. Almost = like. For Melchisedec see ch. 7.