What is the difference between Single and Multi Document? In the case of Single documents, there is only one active document used (for example, a case). When another case is created it closes the existing document (removes the case from clipboard) and open the new case. Single Documents are useful in cases when users work on one case at a time or access the work list to open cases. Single Documents are pretty useful in keeping the application from overloading too much data and the performance is always better. To add a single document, we just add the dynamic container outside the existing layout in the section and then reference a section or harness.
Multi Document mode allows opening multiple documents at the same time. It is added the same way as a single document. The system allows us to have a maximum of 16 simultaneous documents and by default it sets 8.
Also refer below PSC post for getting more clarity: