Here is the explanation from FTOS 9.4 Configuration Guide ...
FEFD (Far-end failure detection) is supported on the Force10 S4810 platform. FEFD is a protocol that senses remote data link errors in a network. FEFD responds by sending a unidirectional report that triggers an echoed response after a specified time interval. You can enable FEFD globally or locally on an interface basis. Disabling the global FEFD configuration does not disable the interface configuration.
The report consists of several packets in SNAP format that are sent to the nearest known MAC address. In the event of a far-end failure, the device stops receiving frames and, after the specified time interval, assumes that the far-end is not available. The connecting line protocol is brought down so that upper layer protocols can detect the neighbor unavailability faster.
If I understand it correctly CISCO's UDLD main purpose is to detect potential uni-directional links and mitigate the risk of loop in the network because STP cannot help in this scenario. Force10 has another feature to prevent a loop in such situation - STP loop guard.
The STP loop guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by a hardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a participating STP link may become unidirectional (STP requires links to be bidirectional) and an STP port does not receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a Forwarding state. This condition can create a loop in the network.