So, I know they're supposed to be pretty similar, but it looks to me that they are more different than it appears at first glance.
In Core your have
1.) Minor Milestones - after every session.
2.) Significant milestones - "Significant milestones usually occur at the end of a scenario or the conclusion of a big plot event (or, when in doubt, at the end of every two or three
sessions)"
3.) Major milestones - A major milestone should only occur when something happens in the campaign that shakes it up a lot—the end of a story arc (or around three scenarios)
Scenario (in Core) is defined as:
As mentioned in Running the Game, a scenario is a unit of game time usually lasting from one to four sessions, and made up of a number of discrete
scenes.
Meaning, Major milestone should happen after between 3-12 sessions (aka 3 scenarios)?
Meanwhile Condensed rule for Breakthrough (which should be equivalent of Major milestone):
An arc is a series of sessions that often contain plot elements that carry
over from session to session. Those plot elements don’t have to conclude
within an arc, but there are usually significant developments and changes that
come about over the course of it. Think of this as similar to a third- or half-
season of a television show. It’s likely comprised of about four sessions of play.
So basically, is Condensed effectively speeding up the advancement a lot?
Also Scenario - Arc difference seems very vague in Core.
For additional confusion there is this difference between Fate Core SRD and actual PDF;
Fate Core SRD:
https://fate-srd.com/fate-core/long-game
Here it says:
An arc is a complete storyline with its own themes, situations, antagonists, innocent bystanders, and endgame, told in the span of a few scenarios (somewhere between two and five, usually).
Meanwhile the rules PDF (p.252) says:
An arc is a complete storyline with its own themes, situations, antagonists, innocent bystanders, and endgame, told in the span of a few sessions
(somewhere between two and five, usually).
Which one is it?