“Eras” represent various stages of development of Anacreon. Prior to 2017, Eras were referred to as Alphas and Betas. There were 7 Alpha and 1 Beta eras before the first named era (which was Era 3). As of 2018, all existing Anacreon games had been updated to run Era 3 code, but retained features from previous eras such as legacy structures and map layouts.
The original Anacreon: Reconstruction 4021 and its Windows port are not considered eras, since they are essentially a completely different game.