Вообще, читать доку очень полезная привычка
The indexed macro syntax of {MACRO<1-9>} is limited to the context of trigger expressions. It can be used to reference hosts in the order in which they appear in the expression. Macros like {HOST.IP1}, {HOST.IP2}, {HOST.IP3} will resolve to the IP of the first, second and third host in the trigger expression (providing the trigger expression contains those hosts).