SQL Developer Data Modeler

Version 3.0.0 build 653

Using the logical design, within the "Relation Properties" > "Cardinality" dialog,
the "Identifying" and "Target Optional" checkboxes are mutually exclusive.
Can somebody explain that to me?

The help defines "Identifying" as:
"Controls whether this is an identifying relationship. When there is an identifying relationship between a parent entity and a child entity, when the relational model is generated, the following occurs in the child table: *the foreign key to the parent becomes part of the primary key of the child.* (In non-identifying relationships, the foreign key to the parent table is just another column in the child table and is not part of the primary key.)"

and "Target Optional" as:
"Controls whether the *target entity in the relation must contain one or more instances*. If this option is enabled, there can be zero target instances; if this option is disabled, one or more target instances are required."

Just because a target entity's PK includes the source entity's PK,
why would that require an instance of the target entity to exist?

I'm trying to model a parent/child relationship where an instance of the child entity can't exist without the parent instance (no orphans) but that shouldn't make an instance of the child entity required.
I'm far from a data modeling guru so thanks in advance to whomever can explain what I'm missing.