The following excerpt from the Flagstaff Arizona Code details the differences between full shielded, partially shielded, and unshielded fixtures. These definitions are universal.
Flagstaff Arizona Code; Division 10-80.20: Definition of Specialized Terms, Phrases, and Building Functions
Fixture, Fully Shielded: A light fixture or luminous tube constructed and mounted such that all light emitted by the fixture or tube, either directly from the lamp, tube, or a diffusing element, or indirectly by reflection or refraction from any part of the light fixture, is projected below the horizontal. If the lamp or tube, any reflective surface, or lens cover (clear or prismatic) is visible when viewed from above or directly from the side, from any angle around the fixture or tube, the fixture or tube is not fully shielded. Examples of fully shielded light fixtures include, but are not limited to, those illustrated below (Note: to be fully shielded these fixtures must be closed on top and mounted such that the bottom opening is horizontal).
Examples of fully shielded fixtures
Fixture, Partially Shielded: A light fixture constructed and mounted such that most light emitted by the fixture, either directly from the lamp or a diffusing element, or indirectly by reflection or refraction from any part of the fixture, is projected below the horizontal. Light emitted at or above the horizontal direction (sideways or upwards) arises only from incidental decorative elements or strongly colored or diffusing materials such as “honey” or colored glass or plastic. Fixtures using spot or flood lamps are considered partially shielded if the lamps are aimed no higher than 45 degrees above straight down (half-way between straight down and straight to the side). Examples of partially shielded fixtures include, but are not limited to, those illustrated below.
Examples of partially shielded fixtures
Fixture, Unshielded: A light fixture that is not fully shielded or partially shielded. Examples of fixtures that are unshielded include, but are not limited to, those illustrated below (Note: even though the lamp in these fixtures is shielded from direct view when viewed from the side or above, reflective surfaces and/or lens covers are directly visible from the side)
Examples of Unshielded Fixtures