This will force mounted troops with lances to use them unless they are in close melee or dismounted. They apply both to your troops and the AI. Un-tick the box to turn the specific fix off. When the box is "ticked" the 'fix' will be active. Here you need to enter the number that you've set in game Options (or with the Warband Battle Size Changer tool) for your battle size if you want to use the deployment feature where you can choose which troops spawn at the start of a battle
You can access the mod options in camp menu (just mouse-over them in game for a tooltip).