Final answer:
Sustained use discounts are automatic discounts applied to Compute Engine resources when used significantly within a billing month, encouraging efficient and continuous usage without upfront commitments.
Step-by-step explanation:
Sustained use discounts are automatic discounts that you get for running specific Compute Engine resources for a significant portion of the billing month. These discounts are applied to the use of virtual machines (VMs) over a certain threshold in a month, incentivizing continuous and efficient use of resources. Unlike upfront commitments or preemptible VMs, sustained use discounts require no prior commitment from the user; the discount is automatically applied as the usage increases.