I am seeing conflicting views on this topic in various posts. When do you mark the 2nd box in FCRA "Info obtained from a third party"? Only when it relates to the credit history of the applicant (i.e., verfication of rent)or any time you deny based on 3rd party info (i.e., denied for collateral due to low appraisal)?

Adverse Action Notice Requirements – Based Upon Third Party Information [§615(b)]:

If adverse action is taken concerning consumer credit (not deposit accounts or other applications) because of information obtained from a person other than a consumer reporting agency (i.e. employer, landlord, other banks, etc.) and the information reflects upon the consumer’s credit worthiness, credit standing, capacity, character, general reputation, personal characteristics, or mode of living, the bank must disclose to the consumer his/her right to know why she/he was denied and that she/he must make a written request for that information within 60 days.

Do not mark 3rd party due to a low appraisal. It has nothing to do with the consumer's credit worthiness.