I was initially searching for some way to aggregate/calculate the total bandwidth of a zabbix-agent, but there is no way to do that through LLD.
So, I wanted to use an intermediate calculated item at the host level (through template) but there is no wildcard support for there (so can't use {#IFNAME*} as a simple example

I tried to then define a user macro {$ETH_PUB_INT} and use that in the calculated item :

Type: Calculated
Item name : net.bw.out
Formula : last("net.if.out[{$ETH_PUB_INT}]")

But then at the zabbix server log, it says :

error reason for "host.domain.com:net.bw.out" changed: Cannot evaluate function "last()": item "host.domain.com:net.if.out[{$ETH_PUB_INT}]" does not exist.

So it seems that it's not expanding user macro to eth0 (as defined at the host level) and from doc (https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/calculated) it seem that "User macros in the formula will be expanded if used to reference a parameter or a constant. User macros will NOT be expanded if used to reference a function, host name, item key or operator."

In my case, it's not an item key, but a paramater for an item key, so shouldn't it be expanded ??