I think is the same I say. At least should be.
You could – free – stay in Era or go TBCC. If yo also wanted other “expansion” you pay for clone.
I protest that almost uniformly people take paid clone as to mean one staying in Era – can just as well be free to Era and pay for TBCC (Later Wrath-C).
It was the other way around:
You could stay (as in follow the flow just like in Vanilla) and progress to TBC, or move to a new clean server called Classic Era.
The difference is small, but it explains why guilds etc. had to be re-created on Era: if you chose Era your character was moved there, rather than staying “as-is” on the now TBC Classic server.
WoW: Classic Era realms didn’t restarted WoW: Classic. This was a spin off project, but it continued from the same point as WoW: Classic (Vanilla) was transitioning to TBC Classic in prepatch. I call it a spin off because you had to take action to participate in Classic Era, while you could do nothing and find your character in TBC and later on in Wrath, just as in any other WoW expansion end/start.
Yes the progress on Classic Era was felt as if it was fresh, but it was so low because all competitive Naxx raids progressed to TBC and thouse who stayed lacked players to form a good Naxx raid, so they rolled back to ZG / BWL progress and stayed there for quite a while untill Era started slowly regrowing.
OK, but still WHEN you choose – on log in – you had two choices: Era or TBCC. Both free. If you wanted other “expansion” as well, you had pay! Cloning was both ways.