Welcome to the feature requests section 😉 All suggestions for features are welcome of course, but I think we already got you covered on this one.
Let’s say you have a data item Course, which has a relation to a data item Teacher and a data item Student. Each Course can have one Teacher and multiple enrolled Students. When you expand your data item Course, you can see the name of each property on the left and the type of the property on the right. The type of property already indicates what kind of relation it is; for the teacher it will simply say “Teacher” (indicating one), while for the student relation it will say “Student list” to indicate that each course can have a list of Students (indicating many). On the Teacher side of things it might say “Course list” if a Teacher can teach multiple courses.
If you’re used to relational databases, seeing types like 1-to-1 or 1-to-many might be more up your alley, but we chose to call it lists because a lot of our users aren’t.