The question is based on chapter 2.2.
Scale-up – exchanging the resource for the more powerful resource. The opposite of this process is scale-down – exchanging the resource for the less powerful.
Scale-out is adding identical resources to the pool – and this is happening when you need the additional resources. By applying the same logic as for scale-up/scale-down, the scale-out process should have some opposite process – so what is the name of the process when the additional resources are not needed anymore and they are being removed from the pool?
You can scale out/in which adds more nodes or you can scale up/down which adds more resources to existing node/s.
Typically scaling out/in is preferred because it increases resiliency and performance at the same time. You can scale out (add more nodes) to meet demand and then scale in (remove under-utilized nodes) when demand is lower.
I hope that helps.