The EXIT_SAPLMHD1_001 exists in the SAP enhancement SAPLMHD1 for calculating and checking the shelf life expiration date.
You can use this exit to modify the data for calculating and checking the SLED.
To use the customer exit, you have to Set Up the Shelf Life Expiration Date Check.
In contrast with modifications, enhancements are always release-insensitive, as they are not made in the R/3 system, but in a name space that is reserved for the customer.
The general procedure for creating enhancements is contained in the enhancement transaction under Utilities -> Online manual.
The documentation on each enhancement is contained in the enhancement transaction under SAP documentation .