Thanks for your response @Retired_mod. I did find the DBU prepurchase plan but that seems to be separated from the "Azure Reserved Instances" concept. My understanding is that there are 2 bills when using Azure Databricks, one for Databricks itself which accounts for the DBU costs, one for the other infrastructure (i.e. VMs) on Azure. Prepurchase can lowers the DBU costs, but Reserved Instances intends to lower the VM costs.
I did see the post mentioned "Databricks supports instance pools that can come from your reserved instances" but want to see if there's additional setup needed to enable that, and if spinning up a cluster can automatically pick up the Reserved Instances without creating a pool.
Thanks!