In particular, it is a form of deterministic jitter which is correlated with the sequence of bits in the data stream.
Depending on characteristics of the signal and transmission topology, previously transmitted symbols can affect the edge cross over time.
This creates a probability distribution for the timing of the signal edge, which is usually recorded in a PDF.
DDJ's PDF is always a series of pulses at the locations where a specific bit pattern experiences a cross over.
Therefore, in order to get an accurate measure of the DDJ in a particular system, a large number of bit patterns must be analyzed (often just a PRBS) unless an analytical solution can be found.