What is lot Number?
A lot number is an identification number assigned to a particular quantity or lot of material from a single manufacturer. Lot numbers can typically be found on the outside of the packaging. For cars, a lot number is combined with a serial number to form the Vehicle Identification Number. (source: wikipedia)
Enabling Lot Number
To enable lot number go to, Settings -> Business Settings -> Purchases
Check the Enable Lot number checkbox and update settings.
Adding Lot number from Purchases
If the lot number is enabled as described above then when adding purchase stock it will display the lot number field for entering the lot number.
Adding lot number from add opening Stock:
If lot number is enabled then when adding opening stock it displays the input field to enter lot number.
Selling products from specific Lot
Refer to the POS selling documentation Lot number description.