For more information refer to the Instructions for 8453-FE:
About Form 8453-FE, U.S. Estate or Trust Declaration for an IRS e-file Return
The ERO is the one who deals directly with the fiduciary and either prepares tax returns or collects prepared tax returns, including Form 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 Check if also paid preparer, in the Use Only section. A paid preparer who is not the ERO must "sign" Form 8453-FE in the space for Paid Preparer Use Only.