Heat lot number in the above image is highlighted in red.
A heat lot number is used as a way to identify certain components of a manufactured item related to heat treatment received. The heat lot number can be used to trace an item back to a specific furnace, year and finally melt number.
Specific industries require heat lot numbers as quality control in case there are ever any issues as a result of production. In most cases the number is stamped onto the product itself using a laser jet printer. In all cases, the number can be found on the items Material Test Report (MTR) sometimes referred to as a Mill Sheet or Mill Test Certificate.
The following video breaks down additional information that can be found on a typical Mill Test Report.
For additional information on heat lot numbers, check out this article.