Paperback 
Customizing Trigger Alert Messages with Macros
Video Lecture
Description
I created the trigger with these settings,
Name : Disk Space Used Above 2GB, Current Value : {ITEM.VALUE}
Severity : Disaster (My users media type for severity Disaster is to receive alerts through multiple medias)
Expression : last(/host-A/vfs.fs.size[/,used])>2G
Other Resources
Units
https://www.zabbix.com/documentation/3.0/manual/config/triggers/suffixes
Macros
https://www.zabbix.com/documentation/current/manual/appendix/macros/supported_by_location