It is the secret code number given out by IRS. Even a Captain Midnight Secret Decoder Ring can not decipher it. If there is ever an issue of not having filed, in the future, this number SHOULD convince IRS that they have the return. 

In the beginning a DCN was created by the software when we filed. Apparently that did not satisfy IRS, so this the newest and greatest thing.

WHEN ALL ELSE FAILS, READ THE INSTRUCTIONS! https://www.irs.gov/pub/irs-pdf/f8879.pdf

6. Enter the 20-digit Submission Identification Number (SID) assigned to the tax return, or associate Form 9325, Acknowledgement and General Information for Taxpayers Who File Returns Electronically, with Form 8879 after filing. If Form 9325 is used to provide the SID, it is NOT required to be physically attached to Form 8879. However, it must be kept in accordance with published retention requirements for Form 8879. See Pub. 4164, Modernized e-File (MeF) Guide for Software Developers and Transmitters, for more details.