Williams Bible Links™ contains Bible cross-references from 70 public domain sources such as Bible commentaries and combines them into an easy-to-use format.
The abridged version contains the most-cited references for each verse.
The unabridged version divides the references for each verse into levels according to citation count, with level 1 containing the most-cited references, level 2 containing the next tier down, and so on. Sometimes a level is empty. This is normal, as level 1 appropriates references from lower tiers if necessary in order to make a full page.
Levels 1 through 4 contain references "confirmed" with two or more citations. This is to minimize false positives -- verses that are not valid cross-references. Level 1-4 combines all the references in levels 1 through 4. Level 5 contains references with only one citation.
The abridged version contains 400,000 confirmed references. Level 1-4 in the unabridged version contains 1.5 million references, while level 5 contains 3.5 million.
The references for each verse are roughly grouped into approximate topics. Topics are separated by a short line of dashes ("-----").
Occasionally, topics will be based on Hebrew or Greek word usage rather than verse concepts. These topics appear to be mistakes until you realize what is going on.