Final answer:
Mess uniforms are classified into mess dress and mess undress, with the former being more formal and worn for black-tie events, and the latter being less formal, used for less formal dinners or events.
Step-by-step explanation:
The two classifications of mess uniforms are the mess dress and the mess undress (or mess dress informal). The primary difference between them lies in the formality and the occasions for which they are worn. Mess dress is more formal and is typically worn for black-tie events, often featuring formal accessories such as bow ties, cummerbunds, and medals. On the other hand, mess undress is less formal, generally worn for less formal dinners or events, and may not require all of the formal accessories associated with mess dress.