This information is part of the Modelica Standard Library maintained by the Modelica Association.

This block describes a unit delay:

// Time domain description
   y(ti) = previous(u(ti))

// Discrete transfer function
           1
   y(z) = --- * u(z)
           z

that is, the output signal y is the input signal u at the previous clock tick. At the first clock tick, the output y is set to parameter y_start.