The DBUG transient data queue has been in the CICSCSD member in the CAGJSRC library since IDMS 18.0. It is used to write debug messages when DEBUG=YES or DEBUG=QTS is specified on the CICSOPT macro or when certain debugging functions are specified as SYSIDMS input in the CICS startup JCL.
These functions are normally only used to diagnose a system problem at the direction of CA technical support.
The default value for CICSOPT is DEBUG=NO. The destination queue is specified on the DBUGDCT parameter, the default is DBUG.
So you don't really need to use it unless you have to turn on debugging.
The queue is defined as deferred so you need to use a CEMT INQUIRE/SET TDQUEUE command to open it.
The queue has TYPEFILE(OUTPUT) and no DSNAME so it will be dynamically allocated to the default sysout class when it is opened, unless there is a DEBUG DD statement in the JCL in which case it will use that.