How to hide descriptive flexfield for specific users?

Hi all,

we have a customization based on informations added on a descriptive flexfield on PO lines, and depending on the information, the customization will start.

These informations can only be added by department A when entering with the PO (the customization is only for this department).

The other departments can't see this flexfield, in order to prevent risks of adding information and start the customization, which should not happen.

But, in the workflow approval, one of the approvers (Department needs to apply a contract template to the PO and before that, he needs to check this flexfield on View mode because he needs these informations to author contract terms.