Era is an assault frigate equipped with two light plasma cannons with decent range. It resides in the middle-ground between corvettes and destroyers. It’s main use case is defending from Destroyers, as its range and speed allows it to outmaneuver them and deal some significant damage, though you need to make in big numbers to pull that off.
Costwise, you can make two of these for every destroyer, so it is a good investment for fighting against them. However, they get easily countered by submarines, against which they have no weapon.
Eras have a higher cost, but stronger armor and weapons then their ARMADA counterpart (Ellysaw).