The short answer is that they must be of the same species.
There are lots of practical problems making successful grafts, but putting these aside (i.e., assuming grafts are done by masters of the trade), the chances of success decline, the less closely related the plants to be grafted are.
Essentially, there are no problems within a species, such as different cultivars of acer plamatum. It is worth noting that there are a few exceptions, however; some varieties of Douglas fir are incompatible, for example.
It’s about even odds (50/50) that species within the same genera can be grafted and it is virtually zero across families (there is an exception between two succulents).
So, as far as your desire to graft acer palmatum foliage on ace campestre, it cannot be ruled to be impossible.
This side is heads, this is tails, call it in the air …
wink, wink, nudge, nudge.
I've seen a number of nice campestre bonsai by Walter Pall and others, but I've not encountered this being attempted before. Post back in 3 to 5 years.
Edit: Fixed typo.