In an ERD what makes an entity “optional” vs “required”?

Question

As I understand it, the white dot indicates "optional" while the black dot means "required".

If so, a Category is required while a Classified is optional.

But what does that actually mean in the database - to say that a Category is required and a Classified is optional?

Answer 1

If the child of the relationship (in the example above set as "classified"), is optional then the column is defined with the database as allowing null records. i.e set to NULL. This means that you can either specify no records as being linked from classified to category, or many records.

Answer 2

In some cases, it could mean the difference between a foreign key being allowed to be NULL or not.

In other cases it might be something you need to check via constraints or application code.

Unfortunately I'm unfamiliar with the notation used in the diagram you post so I can't tell you what it means there.