Sifting algorithm for group generated by a set
One possible explanation is that it is indicating that you should never try to read or use any matrix element $M_{i,j}$ where $i&lt;j$.  "Referenced" might be talking about "de-referencing" a pointer, i.e., reading a memory cell, i.e., reading an entry of the matrix.  Perhaps the text is indicating that the matrix entries $M_{i,j}$ are only defined for $i \ge j$.

I suggest you read the text with this possible perspective in mind and see if it seems consistent with the surrounding context.