The main difference between the Spirit Martial Realm and the Body Refining Realm is that the Spirit Martial Realm is the true beginning of a martial artist's journey. In a way, the Body Refining Realm is not even considered as being part of the martial artist's path. Only when one reaches the Spirit Martial Realm, can they be called a martial artist.
Upon reaching the Spirit Martial Realm, one can absorb a large amount of Heaven and Earth Spiritual Energy, refining it into a small amount of True Qi.
This is why it is called the Spirit Martial Realm.
What sets it apart from the Body Refining Realm is that the Spirit Martial Realm can utilize the power of True Qi. Once they make a move, True Qi flows through their body, making them extremely powerful.