Doubt in WTO Macro's Executable form

Hi,

I'm learning WTO macro's executable form and tried the below simple code to display 'Hello World' message.
Code:

WTOVAR START
BALR 12,0
USING *,12
WTO MF=(E,WTOMSG)
BR 14
WTOMSG DC AL2(WTOEND-*,0)
MESSAGE DC C'HELLO WORLD'
WTOEND EQU *
END WTOVAR

I din't understand the WTOMSG contents i.e. AL2(WTOEND-*,0).
WTOEND-* is referring to which location and why is second parameter here 0?
Can anyone tell me what happened here?