Frame Relay and ATM support a single bit that can be set for QoS but this single bit is actually intended for use related to drop probability. If a Frame Relay device sends a Frame with this bit set to 1, that frame will be considered to be dropped. This is referred to in Frame Relay as the Discard Eligibility bit (DE) or in ATM Cell Loss Priority (CLP) bit.
MPLS defines a MPLS Experimental bit (EXP) that’s intended for general QoS marking.
Be The Packets - CCIE Study Blog 