A relationship is marked as "Optional" on one side (a circle) and "Mandatory" on the other (a dash). This is an example of:

Optionality/Modality

Explanation

The circle (○) and dash (|) symbols placed at the ends of a relationship line in an ERD are the notation used to express optionality (also called modality) — they describe whether participation in a relationship is required or not.

• The circle means zero — the entity on that side is optional, meaning an instance does not have to participate in the relationship.
• The dash means one — the entity on that side is mandatory, meaning an instance must participate in the relationship.

So in the diagram, a Customer must have at least one Order is not guaranteed, but an Order must always be linked to a Customer. That asymmetry — one side required, the other not — is precisely what optionality/modality captures.

 

The other options don't fit: metadata describes data about data, foreign keys are a database implementation mechanism, and normalization is a process for reducing data redundancy in table design. None of those are represented by the circle and dash notation on a relationship line.