IBM 6400 Setup Manual page 301

LU type 0
LU type 0 printers are Non–SNA Local or Bisync connections. This mode is
also commonly called DSC (Data Stream Compatibility) mode printing. An
example of an LU 0 printer connection is an IBM 3816 attached to a
non–SNA 3174 control unit. Not all printers support this type connection.
This connection type is used when VTAM is controlling the non–SNA
control unit and device.
LU type 1
An LU type 1 is for an application program that communicates with single or
multiple–device data processing workstations in an interactive, batch data
transfer, or distributed data processing environment. The data stream
conforms to the SNA Character String (SCS) or Document Content
Architecture (DCA). Examples of printers that are able to communicate via
LU1 are 4230, 4234, 3930, 6400 and others.
LU type 1 is the basic logical unit type for SCS, IPDS, and AFPDS (PSF)
printing. The type of connection is determined by the application that will be
driving the printer. The selection is made by changes in the VTAM logmode
entry that is used for the logical unit. See the examples for the type of
connection desired.
LU type 2
LU type 2 is use by an application program that communicates with a single
display workstation in an interactive environment, using the SNA 3270 data
stream. Type 2 LU's also use the SNA 3270 data stream for file transfer. An
example of an LU type 2 device is a 3270 display such as a 3278, 3178, and
others.
LU type 3
An LU type 3 is for an application program that communicates with a single
printer using the SNA 3270 data stream. Another name for this connection is
Data Stream Extended (DSE). This type of connection will not support the
SCS data stream and may be used when no host controls are to be sent to the
printer. An example of printers that may use this method of connection are
4230, 3912, 6400, 3930, and others.
LU type 4
LU type 4 is is seldom used today. It is used for (1) an application program
that communicates with a single or multiple–device data processing or work
Appendix D. Attaching Host Systems to a Coax/Twinax Printer D–19