TestLink

In TL, when define CF, there is drop down box called 'Available for' and there is option for 'test plan'. When I select 'test plan', dropdown for 'enable on test plan design' is gone. Could someone tell me what it is used for?

The reason I ask this question is that one of my customer field value is same for whole test plan. It will be very terious if we input value for each test case during test plan design.

Thanks in advance,
Yan