From Robert Read:

It's possible for coordinators on different MDTs to produce duplicate cookies in HSM actions. However, checking the cookie is the only way to identify actions (for example, to cancel an action), so it is important for cookies to be unique. This breaks the HSM copytools that assume that cookies are unique.

In order to provide unique cookies, it be helpful to includ MDT index into the action. That will helps HSM copytools to know which MDT an HSM action comes from. And that is particularly helpful for diagnostic reasons.

There are some unused fields in HSM action which can be used to save MDT index.