回答:
それはそのサービスの最後の終了コードです。
launchctl
manページには、それを説明しています。
list [-x] [label]
With no arguments, list all of the jobs loaded into launchd in
three columns. The first column displays the PID of the job if
it is running. The second column displays the last exit status
of the job. If the number in this column is negative, it repre-
sents the negative of the signal which killed the job. Thus,
"-15" would indicate that the job was terminated with SIGTERM.
The third column is the job's label.