About Dstran or Stran of cohesive element in ABAQUS

Sir,
  I want to ask you a question,
  I debug a constitutive model about the cohesive element in ABAQUS.
  You know the key point of this element's constitutive model is Traction-Seperation Relationship.
  I want to test my model by Umat, not the Uel.
  So I want to ask whether Stran is the relative displacement(not strain) of the Gauss point in this type element.
  And how Dstran is calculated by UEL subroutine!!!