You don’t see a supertype-subtype relationship defined as such when you’re looking at the physical database. You’ll only see it explicitly in the logical data model. So what is the pattern and how do you know that you have one in your database?
This relationship exists where you have one entity that could have different attributes based on a discriminator type. One example is a person. Depending on the role of that person in relationship to the business, you will need to store different pieces of information for them. You need different information about a client than you do an employee. But you’re dealing with a person so there is shared information.
It’s a good pattern for minimizing data repetition.