1. to write at the beginning, dictate, order;
2. to lay down a rule : DICTATE;
3. to lay down as a guide, direction, or rule of action : ORDAIN;
4. to specify with authority.
A prescriptive relationship, then, is a relationship in which a person or people create a set of rules governing the relationship before the people involved ever get into said relationship. These rules are often set up to define a relationship involving someone who is not even in existence yet and therefore cannot give his or her input into the relationship agreements.
By contrast, a descriptive relationship describes the relationship as it is at the time with input from all involved.