Outputs

What you get

track_hg38_cas12a/
  cas12a.bb                  11.7 GB   bigBed, 135,867,611 rows x 23 fields
  crisprDetails.tab.gz       78.7 GB   bgzip off-target lists (243.5 GB uncompressed)
  crisprDetails.tab.gz.gzi   60 MB     bgzip index

For the hg38 example, 106,017,484 of 135,867,611 guides (78%) carry a stored off-target list (_offset > 0); the other 22% store none (_offset = 0): non-unique guides, or unique guides already perfectly specific. Uniqueness itself is read from the unique_TTTV / unique_TTTN columns, see Reading the track.

Reading a details record

crisprDetails.tab is two columns: per-bucket mismatch counts, then the |-separated off-target list. Counts are raw absolute totals ([0-mm, 1-mm, 2-mm, 3-mm, 4-mm]), unaffected by chunking.

_mismatchCounts   _crisprOfftargets
0,2,0,0,2         chr2;182255206-;466;117|chr15;101948968-;344;94|chr19;83578+;344;94|...

Each off-target is chrom;pos±;encas12a_cfd;ascas12a_cfd (CFD scores x1000). A guide’s bigBed row points at one such line by uncompressed byte offset (_offset), which the browser resolves through the .gzi.