Why is a field which doesn't exist in _raw data being added to an index?

[vn_g]

Eg :

index = userinformation

_raw doesnt have any field or value related to field "ue".

But "ue" is being shown in Interesting Fields.
ue = abc@splunk.com
ue = xyz@splunk.com

So my questioning is what is generating this field to exist in the index?

[richgalloway]

Look for INGEST_EVAL statements in transforms.conf.

[mykol_j]

forgive my ignorance, but I don't understand the answer.

Do I look on my indexers (all 3 in my case) for this?

What am I looking for?