Field help (ITMOFS) 

For purchasing
This is the number of calendar days, corresponding to:

This information is used for the calculation of net requirements only in order to calculate the start date of sub-contract operation suggestions when the product does not have a product-site record.

For an order direct creation or a purchase request
The reorder lead-time is used to calculate the planned receipt date when no lead-time can be determined from the price lists or product-site record.
This lead-time is also used in the MRP calculation to calculate the requirement start date.