The Select Identifiers step of the Extraction Template wizard captures a component property to uniquely identify items in parts lists.
During BOM Migration, the BOM Migration wizard uses BOM attach to link drawings. It matches the unique identifiers in parts lists with drawings containing the same unique identifier in its title block. Hence, only component properties that are used in both the title block and parts lists are eligible to be used as unique identifiers.
You can optionally select a secondary identifier in addition to the primary unique identifier. The BOM Migration wizard uses the secondary identifier whenever the primary unique identifier is not available. For example, during migration, if the BOM Migration wizard finds a drawing where the title block cell containing the primary unique identifier is empty, it attempts to match the secondary identifier.