Used for design deliverables review and approval:

• when you need to review multiple documents and send a separate response for each document
• each document can be responded to by multiple reviewers from different organizations
• ability to review and markup PDFs through a web browser (PDF Markup Service)
• multiple reviewers from the same organization can markup the same PDF at the same time and see each other's comments and issues as soon as they are made
• helps resend new document versions with fixes

Used for formal Requests for Information (RFIs):

• single response per package from single RFI recipient (cc recipients are kept in the loop but cannot respond)
• no ability to respond per each attached document
• multiple response cycles supported until response is accepted
• ability to suggest and finalize impact to help negotiate scope

Used for all other contractual communication:

• manage and track all contractual communication using a single tool
• multiple responders supported per package - cc recipients can also respond
• threaded view to see multiple related packages and their responses in a single page
• helps track "ball-in-court" responsibility
• better than email: custom form with configurable attributes; response due dates and reminders, audit trail