Spinal Arthrodesis counting

Hello!
I have a question regarding Posterior Spinal Arthrodesis.

Anterior is clearly defined as interspace, Posterior is confusing me a bit.
22630 states Interspace, 22633 states interspace
22600 states single level; and the add on that follows is per segment
How would we count the initial level (22600); by interspace; or by segment?

Thanks so much for your help

Spinal Arthrodesis counting