The PSA framework (introduced in vSphere 4.0) is a collection of VMkernel APIs that allow partners to insert specific functions into the ESXi storage layer.

Those third-party plugins fall into one of three categories:

• Third‐party Multipathing Plugin (MPP): Provides new multipath rules to VMware native multipathing (NMP)
• Third‐party Storage Array Type Plugin (SATP): Used to recognize some storage capabilities, not recognized by the VMware SATPs
• Third‐party Path Selection Plugin (PSP): Similar to the previous one, but usually used to identify the default multipath rule for a new storage

The following image summarizes the PSA architecture: 

SATP rules are used to automatically identify SSD devices (tagged with the SSD type) or local devices. With vSphere 6.5, you can mark a disk as an SSD (or HDD) directly from the HTML5 client:

This can be useful if the storage cannot report the type correctly (for example, if it's a multi-tier storage), and you don't want to build a manual SATP rule.