The Signature ID is any 5-digits of your choice. It is on a screen with today's date and your birthdate.
Your 5 digit signature ID on the Consent to Disclosure screen (also called a Self-Select PIN) is how you sign your federal return if you're e-filing. It's the electronic equivalent of manually signing a paper return.
If the field is blank, just enter any 5-digit
number. You can re-use your IRS e-filing PIN or make up a totally
different number. Same goes for your spouse if you're filing jointly.