It's described in the about screen of the app (this is for version 1.0.0 and version 1.0.1, there might be a slightly improved syntax in future versions):

{property} - Insert property as-is
{property?text} - Insert "text" when property is not empty
{property!text} - Insert "text" when property is empty

(where property is e.g. "song", "battery" or "cell-network" - without the quotes)

Examples:
{song!No song} - Insert "No song" when no song is selected
{date} - Insert the current date (short format)

Advanced features:
{script:uptime} - Insert output of "uptime" command
{script:/home/user/test.py} - Insert output of "/home/user/test.py" script

All the available properties will change from version to version, and they can be inserted with the "Insert" button in the GUI (to be end-user friendly and enhance discoverability). Right now, there is: Compact date, Full date, Song artist, Song title, Song "Artist - Title", Cell network name, Internet connection, Battery percentage, IM presence status, Active profile.

More properties are planned for future versions, but not available in 1.0.0 or 1.0.1 (e.g. upcoming calendar entries, more fine-grained date settings, system information, ...).