I couldn't find it in the docs (i. e., in http://doc.otrs.org/2.4/en/html/x1258.html), http://wiki.otrs.org/ nor in this forum. All that I can find is the footer of the pages:
[ Note ]
Useable options:
<OTRS_CUSTOMER_SUBJECT[20]> To get the first 20 character of the subject.
<OTRS_CUSTOMER_EMAIL[5]> To get the first 5 lines of the email.
<OTRS_CUSTOMER_REALNAME> To get the realname of the sender (if given).
<OTRS_CUSTOMER_*> To get the article attribute (e. g. (<OTRS_CUSTOMER_From>, <OTRS_CUSTOMER_To>, <OTRS_CUSTOMER_Cc>, <OTRS_CUSTOMER_Subject> and <OTRS_CUSTOMER_Body>).
<OTRS_CUSTOMER_DATA_*> Options of the current customer user data (e. g. <OTRS_CUSTOMER_DATA_UserFirstname>).
<OTRS_OWNER_*> Ticket owner options (e. g. <OTRS_OWNER_UserFirstname>).
<OTRS_RESPONSIBLE_*> Ticket responsible options (e. g. <OTRS_RESPONSIBLE_UserFirstname>).
<OTRS_CURRENT_*> Options of the current user who requested this action (e. g. <OTRS_CURRENT_UserFirstname>).
<OTRS_TICKET_*> Options of the ticket data (e. g. <OTRS_TICKET_TicketNumber>, <OTRS_TICKET_TicketID>, <OTRS_TICKET_Queue>, <OTRS_TICKET_State>).
<OTRS_CONFIG_*> Config options (e. g. <OTRS_CONFIG_HttpType>).