Case security (or file security) can be enabled on a per matter basis. It allows you to lockdown a specific matter so only yourself and any specified user can access it. To enable this feature please follow the steps below.
Enter the matter you would like to enable file security on.
Once in, click on the Matter button at the top of the screen. It should look like two stacked squares with a + sign.
Once the Matter screen is opened, in the details section next to Rate Category you should see a tickbox which says Case Security. Tick this to enable it.
Once ticked, a shield icon should appear to the right of it. Click on this to bring up a list of users.
From here you can assign other fee earners to also have access to this file. By default you should already have rights for it.
Once done, cross of the list of users and then on the Matter screen, hit Save at the top.
Note: Supervisors may still have access to the file even if this has been enabled.