A Lot can be used to group Part Records under their own single group Unique Identifier. E.g. your parts are wafers that can be individually tracked using a Unique Wafer ID for each wafer. By using the Lot object you will be able to create a Wafer Lot identifier that can track the group of individual wafers.
First you have to make it possible that a Lot can be created by adding a Lot NV on a Step Definition. This can be done by using the NV Lot Configuration.
Secondly, when you Create a new Lot, you can define the Wafer Lot ID. Now you will be able to create individual Wafer IDs for that lot, or use the Part Grabber to select the wafers you want to add to the Lot.