Every country/culture seems to have a different word for jam or a different way to jam if that use that word.
In the UK, a participatory event is called a “session,” but it’s not really a jam. More of a song circle, and if you play along, you sort of need to arrange that with the person singing before the song starts.
A jam here in les-Hauts-de-France is an amplified event (Rock being KING in this neck o’ the woods –not sure about other parts of France). It starts with a short set from a band and then people in the room are encouraged to get up 2 or 3 at time and play a few numbers before relinquishing their spot to someone else. You Do Not bring your own amp. You can bring your own instrument or borrow one from the organiser, a guy who has carried in and set up the PA, a drum kit, a decent keyboard, a bass, an electric guitar, an acoustic guitar, and all the required sound reinforcement along with a few lights.
(I learned all of this the hard way and speaking very little French. 😉 )
Claudine and I have hosted house concerts in England, and done one in France so far. At these I usually lead a more American version of a jam afterward. I find three chords songs indispensable:
Speed of the Sound of Loneliness
Knockin’ on Heaven’s Door
You Ain’t Going Nowhere
But my guitar player, Julien, is a huge Neil Young fan. So we end up playing
Down by the River