Raja means ROYAL in Sanskrit - Asana means poses. The "Rajasanas" of yoga are Sirsasana (headstand) and Sarvangasana (shoulderstand).
Sirsasana is known as the "King" of all asanas, while Sarvangasana is known as the "Queen" of poses.
Sarvangasana should ALWAYS follow Sirsasana in practice. You can have poses in-between headstand and shoulderstand. Yet, shoulderstand should always be done when a headstand is done.
Sarvangasana can be performed without doing Sirsasana in a practice.
Think of it like this. A King doesn't rule well without his Queen. But a Queen has no problem ruling on her own.
Enjoy the poses.