This is a read-only counter, in milliseconds, tracking how much voice playout was synthesized from parameters or samples preceding in time, using frame-erasure or frame-concealment strategies like those in the G.729 and G.723.1 compression algorithms, due to late or lost data. An admin monitors this to see how much of a call is being filled in by the codec's own loss-concealment logic rather than played from genuinely received data. For example, a call using G.729 with a high value here would show the codec is actively covering for network packet loss using its built-in concealment feature rather than the listener hearing dead air.