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@konrad01 got it pretty close, the Buddhas symbolize various ways Dharma is presented in each of the six realms, expressed so as to match mental disposition of the realm's beings. If you look carefully, on most depictions of Bhava-Chakra the Buddha in each realm holds a different object in his hands. That is the key.
Upvote:1
The wheel of life is full of symbolisms as you probably know, I believe the Buddhas are not actually Buddhas living in those realms, but just a way of showing that the Buddha could see all the realms and also go beyond them, like one who can visit the realms, know the realms and teach about them.
Upvote:2
Ksitigarbha Bodhisattva is famous for being the Buddha who rescues those in hell. The other important personage is King Yama (Yamadevaloka), who is sort of a judge of the dead. These two are a big deal in East Asian Buddhism, I would suppose the Tibetans were aware of them as well.
Ksitigarbha Sutra. I now determine to relieve the sufferings of the beings in the six realms of suffering and sorrow, skillfully leading them to salvation through innumerable kalpha, before I myself attain Buddhahood.