The display of this filter is controlled by below config in FlexNet Operations:

"System"-->"configure"-->"End-User Portal Setup"-->"Portal Navigation"-->"Hide License Support menu"

When "Hide License Support menu" config  is enabled or checked then the filter does not show up and when the config is disabled, the filter shows up. Please refer below screenshot.

The purpose of this filter is for those cases where the user needs to fetch line items with available quantity as zero. If only partial quantity is fulfilled, then this filter will not make any difference.

If available quantity is not 0, it will display the line items even if filter is not selected.