For mediators that prefer to allow clients to choose what documents they disclose to the other side, you can enable this in your account settings (shown below).
Step 1: Go to Account > Settings
Step 2: Click on the heading 'Matters' and scroll down until you see the options shown below. Select 'yes'
That's it. Now through the client portal, your clients will be able to mark their own documents as “accessible” (shown below).