As library staff we would like the ability to customise the email subject heading of Alma letters using letter data so that we can flag something up to a customer/use in their email inbox.

For example, we've recently customised our On Hold Shelf Letter using XSL to have different content when a customer is picking up a Resource Sharing request as we ask customers to collect these from our Customer Services desk whereas other requests are placed on a self service hold shelf in the library.
We were able to customise the letter using XSL, but could not find a way to change the email subject heading for resource sharing requests in the same way as we were able to make the rest of the letter content conditional using XSL.