From 'Nine at the Beginning' to 'chaste and auspicious.' Rectified Meaning: 'The marrying maiden as a concubine-attendant'—a young maiden is called 'mei,' and following as an attendant is called 'gui.'
Nine at the Beginning goes from Dui to Zhen, not being an equal marital match; this carries the meaning of following as an attendant, hence it says, 'The marrying maiden as a concubine-attendant.'
'The lame can tread'—a younger sister succeeding an elder sister as an attendant, though not the primary spouse, does not lose the constant Dao, just like the foot of a lame person: though not upright, it does not cease to be able to tread; hence it says, 'The lame can tread.'
'Undertaking an expedition brings good fortune'—elder and younger are not an equal match; advancing as a principal wife would bring misfortune, but advancing as an attendant brings good fortune; hence it says, 'undertaking an expedition brings good fortune.'
Commentary note from 'In continuing the ancestral succession through the ruler's son' to 'good fortune, which is fitting.'
Rectified Meaning: 'In continuing the ancestral succession through the ruler's son, though young, he does not act rashly'—this establishes an analogy for the young maiden.
It means the ruler's son ought to inherit the succession, just as a consort's younger sister ought to become an attendant. Establishing succession usually prefers the elder, but if the ruler's son is established though young, it is not rash.
In terms of marriage, an equal match is desirable, but though the consort's sister is very young, going as an attendant is viable.