.daBriefcase files are a component of the Briefcase feature of Data Abstract which provides a means for clients to serialize client-side data and change deltas to a file on the client file system.
A .daBriefcase uses a binary file format that corresponds to the Bin2DataStreamer format. Data Abstract includes a tool to read (and in the case of the Windows tool; edit) the data stored in the .daBriefcase file. If you are using a Windows based version of Data Abstract you will the Briefcase Explorer tool. On Mac OS X the .daBriefcase can be opened in Server Explorer.