The processes in the viewers category are dedicated to display data. Each viewer is used for a specific data type (for instance Raw T1 MRI, or White matter mesh, ...).
Automatic viewers have the ability to be automatically triggered by the graphical interface system of BrainVISA: the "eye" icons displayed on process parameters. To be valid, an automatic viewer must have only one mandatory parameter (the first one), corresponding to the data to be displayed. Additional optional parameters are of course allowed. The viewers list is open: you can add new viewers, and BrainVISA will be able to use them and display "eye" buttons on process parameters representing the corresponding data types.
Most of the viewers currently shipped with BrainVISA use Anatomist. This is not mandatory: it is perfectly possible to write a viewer using a text editor, a curve plotter program, or whatever.