I've had trouble wrapping my head around the question when to use 1-to-many relationships in Marketing Cloud Contact Builder. I know as theoretical concept you should use 1-to-M relationships if e.g. connecting a customer data extension with orders data extension (i.e. one customer can have many orders). However, unlike 1-to-1, what does it mean in practice? Am I mapping multiple fields in such instance or ? I've posted a screenshot of an example from a SF education.
table = dataextension
I am not 100% sure if i got your question right but will try to describe it with the screenshot you provided.
First thing you should understand is the difference between a primary key and a foreign key. PrimaryKeys are used to identify a uniqueness in a particular table. Foreign keys are used to define relationsships between tables. Imagine you want to search all orders from a particular customer. Then you make a relationship from Contacts to Orders which would be 1 contactkey inside the table "ContactKey" can have multiple orders inside the Orders table.
When a relationship is marked as 1 to 1 there can only be one record in for each contactkey. That is commonly used when you would relate static values to a customer like prename, surname, ... . Meaning you have a table / dataextension with many fields like those i have mentioned and one of them is customerID which will be related to the contact key.
When you want to store multiple records for one customer you need to have a 1:M relationship. If you would have set this to 1:1 each contact could only have 1 order (with all the fields of that particular order).
Does this clarify your question?