how to define the factor of damage due to ductility

[ocean2000]

the Hysteretic Material model in the manual:
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uniaxialMaterial Hysteretic $matTag $s1p $e1p $s2p $e2p <$s3p $e3p> $s1n $e1n $s2n $e2n <$s3n $e3n> $pinchX $pinchY $damage1 $damage2 <$beta>
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$damage1--->damage due to ductility: D1(mu-1)
what's the mu stand for? and the (mu-1)?

please give me some docunment about thease factors:$damage1,$damage2 and the the Hysteretic Material model

thanks all