POID - Page Object ID
PPID - Previous Page Object ID
COID - Current Object ID
OOID - Organisation Object ID
DOID - Domain Object ID
HOID - Home Page Object ID
UOID - User Object ID
ONUM - Organisation Number
ONAM - Organisation Name
DNUM - Domain Number
DNAM - Domain Name
You can use any of the above surrounded by {} to get the value. So {HOID} will give you the ID of the home page.
Also see {SYS_VAL} command.