Open source software is practically the same as free software, with only a handful of deviations:
In “Why Open Source Misses the Point of Free Software,” Stallman explains: “The two terms describe almost the same category of software, but they stand for views based on fundamentally different values. Open source is a development methodology; free software is a social movement.”
FOSS is just the term for both groups together (Free and Open Source Software).
You have it backwards. Free and Open Source software is Open Source (subset). But Open Source is not Free and Open Source (superset).
Langfuse is a great example of where this is the case: https://github.com/langfuse/langfuse/blob/main/LICENSE
It is open source, but all features under the ee folder are not free, thus it is not FOSS.
From reviewing the license, the portions under the ee directory are not open source, they’re source-available with some additional grants of rights given certain conditions.
Here’s the definition I use for “open source”, and here’s the one I use for “free software”. Most (all?) free software licenses meet the definition of free software, but not all open source licenses meet the definition of free software, so that’s why I tend to set that free software is a subset of open source software.