I'm 99% sure that the conversion rate is the share of the leads in the stage that has continued to the next stage in the success path.
In the below example, the SQL stage conversion rate x the number of leads that entered the SQL stage (the "in flow"), should equal the "in flow" of the Opportunity stage, because that is the next stage in the success path. Because there's only one stage that leads in SQL can move to, the "out flow" of SQL should almost equal the "in flow" of Opportunity.
I write almost, because leads can sometimes jump from one stage to another, if you change the revenue stage in a smart campaign flow step, instead of letting the revenue cycle model do its job.
Calculation example: "In flow" of SQL stage is 900 leads. Conversion rate i 25%. Both "Out flow" of SQL as well as "In flow" of Opportunity should then (approximately) equal 25% * 900 leads = 225 leads.
The same thing goes for the Opportunity stage, where leads can move both to Lost and Won. "Out flow" will be the sum of the leads that have moved to those two stages, while conversion rate will only tell us how many leads, who have moved the next stage in the success path - and in the below example that would be the Won stage:
