"-1" means that cron should index all items at once.

On the edit tab for an index there is an option under Index Options to set the "Cron batch limit". In the directions it says, "Set how many items will be indexed at once when indexing items during a cron run. "0" means that no items will be indexed by cron for this index, "-1" means that cron should index all items at once." However, if you enter -1, then on the view tab it says, "Cron batch size: No items will be indexed during cron runs." when I would expect it to say that it would index all items at once.