What is the difference between a subnet mask and a wildcard mask?

Hello, nice to meet you.

As an example, the combination of the two seems to be used in combination to provide IP-based access control (ACL) on CISCO's network equipment.

The subnet mask must have consecutive bits from left to right (RFC 46323.1), and the wildcard mask does not have this specification.This allows you to control a subnet with an odd number of octets, such as communicating.

Whether it is necessary or not is probably subjective, but the existence of an implemented processing system may mean that there are cases where it is necessary in practice.

https://www.rfc-editor.org/rfc/rfc4632#section-3.1
https://www.cisco.com/c/en/us/td/docs/app_ntwk_services/waas/waas/v511/command/reference/cmdref/ext_acl.pdf