Undefined

This might not sound like a tip to you. You might think, “Dimensions won’t get sent? Sounds like a bug!”. You’re wrong. This is one of the awesome features of the GA API, and it’s key to making your tag setups leaner when sending data to Google Analytics.

Tip 4: Undefined dimensions are left out of GA hits

Note that ‘undefined’ here means the special value undefined in JavaScript. For a refresher, check this post. If you use a macro in a dimension field of Google Tag Manager’s Universal Analytics tags, this dimension will be left out of the hit payload if its return value is undefined.