According to sessions pallet docs:

Session: A session is a period of time that has a constant set of validators. Validators can only join or exit the validator set at a session change. It is measured in block numbers. The block where a session is ended is determined by the ShouldEndSession trait. When the session is ending, a new validator set can be chosen by OnSessionEnding implementations.

And on this site:

An era is a period of time during which there is a specific set of active validators. Each era has 6 epochs (or sessions). Before the last epoch the active set of the next era is elected. And after the end of each era the rewards are calculated and are ready to be distributed to the validators and nominators. An era is 24 hours on Polkadot and 6 hours on Kusama (give or take a few seconds).

So as I understand either the set of active validators in an Era is a superset of that which is used in one Session of which still smaller subsets may be rotated in an Epoch if defined to be smaller than a Session. On Kusama Epoch and Sessions are of equal length.

Will I be correct in understanding that each session rotates based on BABE VRF from a set of active validators for the current Era? Or is it the same set for all sessions of one Era?

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The validator identities are the same throughout an entire era, i.e. new validator elections only happen once per era, but every session the validators can use new session keys (https://docs.substrate.io/v3/concepts/session-keys/), but these session keys are still tied to the same validator identities.

Epoch and Session are essentially synonyms, BABE just uses the epoch terminology since that's what is used in its paper.

I think learning about stash / controller / session keys might help with understanding what I mean by "validator identities" (https://wiki.polkadot.network/docs/learn-keys).

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