The XT5LU340AEFF000XXX is a 400 A, 600 VAC, 3-pole molded case breaker in the ABB XT5LU series, fitted with an EKIP LS/I electronic trip unit and front terminals on a 400 A frame. The 600 VAC rating covers line-to-line voltages up to that level. The 3-pole configuration handles three-phase circuits. The EKIP LS/I trip unit provides adjustable long-time and short-time protection, plus instantaneous pickup.
The closest peer is the XT5NU340ABNF000XXX, also a 400 A, 3-pole molded case breaker but rated for 690 VAC instead of 600 VAC. The XT5NU variant uses a different trip unit (ABNF vs. EKIP LS/I) and likely a different frame suffix. If your panel was specified around the XT5NU340ABNF000XXX, the XT5LU340AEFF000XXX will not drop in without checking the trip-unit coordination and voltage rating — the 600 VAC limit is the hard ceiling. For a 480 V or 600 V system the XT5LU works; for a 690 V system you need the XT5NU.