The Declaration Control Number was a unique number assigned to every electronically filed tax return. The DCN couldn't duplicate a previously e-filed return, or the IRS would reject the return.
However, the DCN has outlived its usefulness. Prior to 2012, a DCN was automatically assigned to each federal income tax return to show that the taxpayer with a given Social Security number had already filed a return. The DCN consisted of the taxpayer's individual electronic filing ID number, a batch number, and a number signifying the order in which the return was submitted.
The DCN prevented many cases of fraud by rejecting duplicate attempts to file, according to Top Tax Defenders. Any taxpayer or tax preparer e-filing could receive notifications about issues with the return through the DCN.
After the 2011 tax year, the IRS updated its electronic filing system, which eliminates the use of DCNs.