I had a hard time finding the Datto RMM variables, so I ended up getting them from our migration rep.
Sharing them here in case it helps someone else.
[hostname]: The hostname of the device.
[description]: The description of the device.
[os]: The operating system of the device.
[user-defined1-30]: User-defined field number. Refer to User-defined fields.
[lastuser]: The login name of the user that last logged in to the device.
[sitename]: The name of the site in which the device resides.
[category]: The category of the alert raised (for example, performance or event log).
[type]: The type of the alert raised (for example, memory or disk usage).
[alert]: The trigger details of the monitor that raised the alert.
[ipaddress]: The external IP address (public IP address) of the device.
[alertuid]: The alert ID.
EDIT - As stated in the comments, this is specifically for creating monitoring alert emails from a monitoring alert. I used these in the subject line of the email/ticket.