Difference between issue and issueFunction in JQL

[Manel]

Hello,

I need to identify all stories without tasks having link type "is tested by" so  I created this filter (syntax was accepted):

issuetype = Story AND issue not in hasLinks("tested by")

But when I changed issue to issueFunction I got a different results (syntax was accepted as well). 

So my question is: what is the difference between issue and issueFunction here?

Thank you.

[Nic Brough -Adaptavist-]

"Issue" is literally the issue.  Most people using Jira would look at it and expect to see the issue key (and they would, although what you see is not quite what it really is)

issueFunction is a (sort of) custom field created by ScriptRunner to accumulate a load of issue information together in the index to prevent it having to do massive calculations every time certain functions are used.