The ATTACH command is used to attach your terminal to a job.  Examples:

    .ATT 27 [1,PRG]	Attaches job 27 belonging to [1,PRG].  You can do
			this without first logging in, but you must type a
			password unless you are logged in as [1,PRG].

    .ATT 27		Attaches job 27.  This only works if you are the same
			user as that of the job being attached.

    .ATT		If you have exactly one other detached job, this
			command attaches it.

If you are logged in when you give the ATTACH command, your job is detached.
Several commands are related to ATTACH:

    .DETACH		Detaches the current job from your terminal, leaving
			you not logged in.

    .FORK		Creates a new job, attaches it, and detaches the
			previous job.

See the Monitor Command Manual, online as MONCOM.BH[S,DOC], for more details.