A deficit notice is the IRS legal definition of a taxpayer’s tax deficit; the formal name of the form is IRS Notice CP2319A: Deficit and Tax Increase Notice.
A shortfall notice is issued when the IRS proposes to amend a tax return because they have found that the information on the return does not match their records.
A shortfall notice is usually triggered by tax information obtained from a third party, such as an employer or financial institution, that does not match the information provided by the taxpayer.