IBM 6400 Setup Manual page 327

Disable. The default. The printer waits for all print data in the chain to
print before issuing a print complete status. The next print job is not
started until printing of the previous job is completed. This can cause
the printer to print at less than rated speed but can help ensure that all
data is printed.
Enable. The printer does not wait for all print data in a chain to print
before issuing a print complete status. The next job can be started before
printing of the previous job is completed. This can cause the printer to
print at rated speed, especially when many short are sent to the printer at
the same time.
NOTE: Some data, particularly the last few lines of the last print job, may
be lost when this setting is enabled with host software other than
PSF or InfoPrint Manager.
Undefined Character Substitution (Coax IPDS)
Undefined Character Substitution is accessed under the Coax Interface menu
selection. This configuration value applies only to IPDS on Coax. Undefined
Character Substitution specifies the replacement character to print in place of
any unprintable character that is received by the host.
The character code point can range from hex 40 to hex FE. The factory
default is set to hex 60.
Appendix E. Code V, IGP, and IPDS Menus E–25