Question Indicator 16

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The company I work for has several denials from Regence Federal for no indicator "16" on the HCFA. This must be added to the claim for when the claim is submitted electronically. This supposedly indicates they are the secondary or "next payer" as the rep put it. Without this indicator the claim goes unpaid. I have NO IDEA what indicator 16 is. I have searched my coding and billing materials, searched the forums, and various google searches. Can anyone please guide me in the right direction. I am stumped. Thank you!