Per IRS Form 8453-FE U.S. Estate or Trust Declaration for an IRS e-file Return page 2:

The ERO is one who deals directly with the fiduciary and either prepares tax returns or collects prepared tax returns, including Forms 8453-FE, for fiduciaries who wish to have the return of the estate or trust electronically filed. The ERO’s signature is required by the IRS.

A paid preparer who is also the ERO checks the box in the ERO’s Use Only section labeled “Check if also paid preparer.” A paid preparer who is not the ERO must sign Form 8453-FE in the space for Paid Preparer Use Only.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.