How to enable context LOV in requisition page

Summary:

We need to enable context sensitive DFF in requisition page. When user enter the requisition, they can choose which context.

Content (required):

1. I define DFF in requisition page. I create 2 context sensitive segment "Default Context" and "PROSTHESIS".
2. Setup the Configure Requisitioning Business Function and enter context value "PROSTHESIS" in line level.
3. Then go to sandbox to enable the DFF.
4. When I enter the requisition, context is read only and default to "PROSTHESIS". We are not able to change it to "Default Context".

Is there a way that it can allow user to choose context when they enter the requisition? One more thing is we have only 1 BU and need multiple context DFF.