Allow sub-events spaces

Each workshop would have to be a different item to which a registration field is attached. You could create a Meeting entity, and have it relate to N Workshop entities. In this data model, the Registration field needs to be added to the Workshop entity and not the Meeting entity. Sounds like a neat application! We can keep this as a Support Request since your idea is supported already and does not require a new feature.

Hey John, I'm pinging on this as I've finally got some time to continue this project.
Sounds like a nice idea, but that means you can't have any custom fields for the registration on the global "Meeting" entity.

In my case, I've got an "Meeting" event.
Users can register to that event and should give infos like name and phone number
then they should choose between 2 different workshops in the morning (it's either one or the other)
choose between two different workshops in the afternoon (either one or another)
then they can also register for the dinner if they want to (and there are three different types of meals to choose from with different prices)
and they can reserve a parking spot for the whole event.

There should be a price for the global "meeting" event, but also prices to add up for the workshops, dinner meals and parking spot.

And, in an ideal world, I should be able to fix a maximum number of spaces for each workshops and for the whole meeting event.

I understand this is quite a very specific use case, though. ;)

You may need to look into using Commerce Product Attributes. Agree, for something that complex, custom code will likely be a necessity.