As per ICAO doc
EDTO alternate An en‐route alternate aerodrome that
is designated in a dispatch or flight release for use in the
event of a diversion during an EDTO flight, and which
meets the applicable dispatch minima (weather and field
conditions). This definition applies to flight planning and
does not in any way limit the authority of the
pilot‐in‐command during flight.
Note.— En‐route alternate aerodromes may also be the
take‐off and/or destination aerodromes.