macros/variables can be used as a part of item key parameter, for example :

key[a_{HOST.HOST}_b]

that does not seem to be documented, though.

the best place could be https://www.zabbix.com/documentation/3.0/manual/config/macros

if this changed at some point (for example, if macros were only supported as full item key params), we should reflect that in the docs back up to 2.0 and check whether it is mentioned in whatsnew, too