for the purposed of LLD it can some times be required to run an external script against an interface.

say for instance I want to find all the items i can monitor via IPMI I would need to provide the external script with the IPMI interface that is configured on the host consistently.

I could use a macro like {interface.ipmi[1-9]}

for JMX {interface.jmx[1-0]}

so my ipmi discover would have 

ilo_discovery.pl[\{interface.ipmi},\{interface.ipmi.user},\{interface.ipmi.password},sensor,"power supply",numeric]