for reference… it appears this is a fairly limited* set of commands.
in the Help section of Email Subject, on the same page, we find:
Saving with a blank value will restore the default.
Available Variables for Email Subject and Body: {{CALLUID}} - Call’s Unique ID {{ROUTENAME}} - Outbound Route’s name {{DIALEDNUMBER}} - Destination number as it will be sent to the trunk {{DIALEDNUMBERRAW}} - Destination number as it was dialed {{CALLERNAME}} - Caller’s name {{CALLERNUMBER}} - Caller’s number {{CALLERALL}} - example: “CALLERNAME” <CALLERNUMBER> {{OUTGOINGCALLERIDNAME}} - Outgoing CallerID name {{OUTGOINGCALLERIDNUMBER}} - Outgoing CallerID number {{OUTGOINGCALLERIDALL}} - example: “OUTGOINGCALLERIDNAME” <OUTGOINGCALLERIDNUMBER> {{TRUNKNAME}} - Trunk name {{MONTH}} - 2 digit month {{DAY}} - 2 digit day {{YEAR}} - 4 digit year {{TIME}} - example: 17:30:00 {{TIMEAMPM}} - example: 5:30:00 PM {{TZFULL}} - Full Timezone. ex. America/New_York {{TZSHORT}} - Short Timezone. ex. UTC, PST, +12