This parameter simply prevents ‘runaway’ barcharts if you accidentally plot a silly amount of data (for example you forgot a ‘where’ setting on a big database, or omitted a grouping command):

chSet 'bl' 5000 

The default is set to 1000, which is the total number of data items above which barcharts will simply generate a blank page with an error message.

See also ...
TowerLimit – towerchart limit