This option is only available if this is a serialized or header item (i.e. something has been entered the header field). When selected, all the serialized items will be displayed to choose from. The screen looks like the following:

Bulk Serialization – This option is only available if this is a serialized item and bulk is checked and set to Bulk Serialized. All bulk items can be serialized with a unique id. The screen looks like the following:
Note – This is very useful when using Contract Fulfillment and prepping in/out items. Now you bulk items can be tracked to who you sent them out to. A unique id can be a barcode to make the prepping process easier.
Show Header – This option is only available if this is a serialized item. When selected, the header item will be displayed in place of the serialized item.