7.3 Job Control Variables
This variable controls how the shell interacts with the user and
job control. If this variable exists then single word simple
commands without redirections are treated as candidates for resumption
of an existing job. There is no ambiguity allowed; if there is
more than one job beginning with the string typed, then
the most recently accessed job will be selected.
The name of a stopped job, in this context, is the command line
used to start it. If this variable is set to the value 'exact',
the string supplied must match the name of a stopped job exactly;
if set to 'substring',
the string supplied needs to match a substring of the name of a
stopped job. The 'substring' value provides functionality
analogous to the '%?' job ID (see section 7.1 Job Control Basics).
If set to any other value, the supplied string must
be a prefix of a stopped job's name; this provides functionality
analogous to the '%' job ID.