ENDBOX - lifelike/countersheetsextension GitHub Wiki
If a row in the CSV data file contains only a single cell in the leftmost column saying ENDBOX that means no more counters will be generated for the current box.
A box is by default the entire page (minus any Registration Marks), but using a cs_layout layer it is possible to create sheets consisting of several smaller boxes instead to have more control of where counters end up.