The IRS explains the guidelines and provides FAQs for preparers using the Practitioner PIN Method for Forms 1040 and 4868 Modernized e-File (MeF) on its website:

The Self-Select PIN Method ( Personal Identification Number ) is one option for taxpayers to use when signing their electronic Form 1040 and Form 4868. The PIN is any five numbers (except all zeros) the taxpayer chooses to enter as their electronic signature. Taxpayers are required to use a PIN when they file an electronic tax return using an Electronic Return Originator (ERO) or commercial-off-the-shelf (COTS) tax preparation software.

Most taxpayers are eligible to use the Self-Select PIN Method. To do so, taxpayers must provide their date of birth and Adjusted Gross Income (AGI) amount or the Self-Select PIN from the original prior year tax return so the IRS can authenticate them. Click the IRS.gov web page link Validate Your Identity After Signing Your Electronic Tax Return for more information.

If the taxpayer agrees, it is acceptable for an ERO and/or software program to generate or assign the taxpayer PIN. The taxpayer consents to the ERO’s choice by completing and signing an IRS e-file signature authorization containing the intended taxpayer PIN. The taxpayer PIN can be systemically generated or manually assigned into the electronic format and/or the signature authorization form. However, the ERO must receive the signature authorization signed by taxpayers before they transmit the return or release it for transmission to the IRS.

The website also lists General Q&A's for Electronic Signature (PIN) Methods.