In our school year 7 are set for English Maths, PE and Science. In year 8 and 9 they add MFL to that.
It means the children are in different classes.
However, our school is a small school - 122 in each year - and some subjects are paired and this influences the set chosen. For example, my son is in a higher Maths set than he should be because he is top set science and those two are paired. However, my daughter was always in a lower MFL set than she should have been because her English set determined her MFL.
So, after all that, I would say ask the school what their actual setting means in practice.
Assuming you mean in secondary school, it usually is done per subject, so a person might be in a high set for maths and a more middling one for French, or vice versa. And each set will be a separate class.
So your child would have one group of people in their registration class (their main class for most subjects) but then be in a maths class with a mixture of pupils from all the registration classes.
You would need to ask the school what subjects they have sets for, starting at what age. But maths is often set quite soon because they can tell quickly who is already doing well at it. Foreign languages are often set after the school have a chance to see who is picking it up quickly. Other subjects may not be done in sets until they get to choosing subjects for exams.
i think it varies from school to school. at dd's secondary school she will be put in sets for maths, english and science (they sort the sets outs by december in y7 - they prefer to work the kids out for themselves rather than relying on sats) all her other lessons she will have with her class - which is decided randomly and stays the same throughout school. I think they do more setting - bu subject - when they start gcses.
I did have a conversation with someone who told me that there's a difference between "setting" and "streaming". one is by subject; one is for everything. tbh I can't remember which way round they are.but I think setting is by subject
hatwoman - yes "setting" is per subject, "streaming" is for everything - which I don't think gets done much (at all?) any more. Streaming would put a maths genius in the top English set even if he/she struggled with it.