what is stored access policy ?

A stored access policy provides an additional level of control over service-level shared access signature on the server side.

Establishing a stored access policy server ti group shared access signatures and to provide additional restrictions for signatures that are bound by the policy. You can use a stored access policy to change the start time, expiry time or permissions for a signature, or to revoke it after it has been issued.

Note:

Note that a stored access policy on a container can be associated with a shared access signature granting permissions to the container itself or to the blobs it contains. Similarly, a stored access policy on a file share can be associated with a shared access signature granting permissions to the share itself or to the files it contains.

reference:

https://docs.microsoft.com/en-us/rest/api/storageservices/define-stored-access-policy