Why is it necessary to select the policy structure type before creating a storage policy?

Thick or thin provisioning is a disk format setting configured at VM or datastore level and is not determined by the storage policy structure type.

VM virtual hardware version is a compatibility setting tied to the ESXi host version and is completely unrelated to storage policy configuration.

The policy structure type - such as tag-based placement, datastore-specific rules, or vSAN rules - controls which rule categories appear in the policy editor. For example, selecting a vSAN structure type exposes vSAN-specific capabilities like failures to tolerate and stripe width, while a tag-based type exposes tag categories. Without selecting the structure type first, the appropriate rule categories cannot be surfaced for configuration.

Storage I/O Control (SIOC) is a datastore-level feature configured separately in vCenter and is not what the storage policy structure type controls.