When checked, it indicates that the existing part substitutes
Data has to be copied when "Same-As Adding" is done.
When unchecked, it indicates the substitutes
Data must not be copied when "Same-As Adding" is done.
Depending on whether a standard- or Project specific BOM is being created, and whether the project allows project specific substitutes only, the data will be copied to either the Product structure substitutes table or to the Contract substitutes table.