I have never seen this claim that Upasana is a division of the Vedas.
As such, they [Vedas] are divided into four major parts, and each of
these, in turn, is further divided - as may be conveniently seen in
the following table:
Work [karma kanda] : Samhitas, Brahmanas, Aranyakas;
Knowledge [Jnana]: Upanishads;
The Spiritual Heritage of India by Swami ...