Halliday & Matthiessen (2014: 375, 380-1):
The ordering Numeratives (or ‘ordinatives’) specify either an exact place in order (ordinal numerals, e.g. the second train) or an inexact place (e.g. a subsequent train). …
An ordinal is a kind of superlative cardinal: third = ‘three-est’; that is, identified by being at number three.