How to understand "IS EMPTY"

[xl-better]

I found that there is not much info about "IS EMPTY" in nebula docs, i think it's used for variables which does not exist, and "IS NULL" is used for variables which exists but with invalid values.I am not sure if this understanding is correct, can someone help answer this?

[czpmango]

NULL is a value on the property site, and EMPTY reveals that there is no value on the property site.