https://www.zabbix.com/documentation/1.8/manual/web_interface/configuration :
Unit blacklist

By default, specifying a unit for an item will result in multiplier prefix being added - for example, value 2048 with unit B would be displayed as 2KB. For a pre-defined, hardcoded list of units this is prevented:

ms
RPM
rpm
%
Note that both lowercase and uppercase rpm (rpm and RPM) strings are blacklisted.

I think it should be mentioned on https://www.zabbix.com/documentation/2.2/manual/config/triggers/suffixes