Cloud Services - What's the Difference?

Twitter wanted their BigQuery queries to return results within one minute. They did this by allowing internal customers to reserve a number of slots. A slot in GCP terms is it's a unit of computational capacity to execute a query. If you reserve VMs, you pay a lower price for it because you're guaranteeing those cloud services that, hey, you're going to buy this much per month,. Whereas if you're doing spot pricing, you might use it way less, but they're going to jack the price up on you.