Simple sentences, both two-member and one-member, can be divided into below categories based on their complexity:

• Unextended
• Extended

A sentence consisting only of the primary or principal parts is called an unextended sentence.

She is a student.
Birds fly.
Winter!

An extended sentence is a sentence consisting of the subject, the predicate and one or more secondary parts (objects, attributes, or adverbial modifiers).

The two native women stole furtive glances at Sarie. (Abrahams)
The two white overseers… had gone into the hills with the natives to look for stray sheep. (Abrahams)