In these Seven Valleys Bahá’u’lláh describes the Valleys and Cities in reference to three spiritual stations: the station of limitation (tahdíd), comprising the first three valleys; the station of unity (tawhíd), covering the next three; and then the station of annihilation of self and subsistence in God (faná az nafs va baqá bi-lláh).
In the station of limitation the wayfarer understands Truth through the medium of appearance and manifestation. It is in this station that one talks about the differing stations and ranks among the Prophets of God, and that one should not confound them (Gems is even more emphatic on this point). This is when “when the wayfarer gazeth only upon the place of appearance”.
In the station of unity the wayfarer understands that all the Manifestations reflect the same Light, and bring the same essential Purpose and Message. These are those who “gaze upon the effulgence of the light”.
In the station of annihilation of self, one ceases entirely to judge the Manifestation by human standards and limited concepts (“For whatever the creatures have is limited by their own limits”), and instead beholds Them all as being the same Soul, the same Spirit, and uttering the same Word, with no distinction to made among them. These “have drunk of the wine of oneness and… see nothing but the sun itself”.
I read the differing planes as being the answer to this question, posed by Bahá’u’lláh in Gems:
“Know then that it behoveth thine eminence to ponder from the outset these questions in thy heart: What hath prompted the divers peoples and kindreds of the earth to reject the Apostles whom God hath sent unto them in His might and power, whom he hath raised up to exalt His Cause and ordained to be the Lamps of eternity within the Niche of His oneness? For what reason have the people turned aside from them, disputed about them, risen against and contended with them? On what grounds have they refused to acknowledge their apostleship and authority, nay, denied their truth and reviled their persons, even slaying or banishing them?”