EER Design specification I just started learning about Databases, and my first assignment has information about an Art Museum that we are supposed to use to define a DB for, demonstrated with an EER Diagram.

One specification is as follows:

"The museum has a collection of ART_OBJECTS. Each ART_OBJECT has a unique Id_no, an Artist (if known), a Year (when it was created, if known), a Title, and a Description. The art objects are categorized in several ways, as discussed below."

What role, if any, does "if known" play in terms of the diagram? I reviewed the instructors notes looking for something resembling such specification, but I couldn't find anything in the diagram components that would imply whether or not information is available. Thoughts?
In the logical model it means that the attribute is optional, which translates into a nullable column in the physical implementation.