The DAT SCSI 3,25" 4/8GB Systems and Options Catalog is published quarterly as an ordering and configuring publication for DAT SCSI 3,25" 4/8GB customers and sales representatives. It contains a collection of the most current configuring and ordering information available for actively-marketed DAT SCSI 3,25" 4/8GB systems, hardware options, software, and services. The terminology described in this introduction is used throughout the catalog. Model numbers are used to order products in this catalog. For products that are voltage/frequency dependent, the Model numbers appear in the following se- quence: the DAT SCSI 3,25" 4/8GB variation of the model appears first; the 240-Vac/ 50-Hz model follows. For example, with model number RA81 AA/AD, - RA81 AA designates the 120-V/60-Hz model and RA81-AD designates the DAT SCSI 3,25" 4/8GB model. The same rule applies for model numbers printed using parenthesis instead of a slash, i.e., DAT SCSI 3,25" 4/8GB is identical to RA81 AA/AD. - Five factors must be considered when configuring PDP 11 systems - UNIBUS System Expansion Space — The physical space in a CPU or expansion box that accommodates a specific type of backplane or module. For example, SU (System Unit) or quad slots.