Você está "marcando" um painel:
-m and -M are used to set and clear the marked pane. There is one marked pane at a time, setting a new marked pane clears the last. The marked pane is the default target for -s to join-pane, swap-pane and swap-window.
Certas ações agora segmentarão o painel marcado por padrão. Aqui está um exemplo de script bash para testar. Você pode executar este script a partir de uma sessão do tmux.
# /usr/bin/env bash
set -euo pipefail
# Make three vertically split windows with text in each.
tmux split-window -v
tmux split-window -v
tmux select-layout even-vertical
tmux send-keys -t 0 'echo pane zero' C-m
tmux send-keys -t 1 'echo pane one' C-m
tmux send-keys -t 2 'echo pane two' C-m
# You can now swap the current pane with an explicitly targeted pane. Here, we
# change pane ordering from 0-1-2 to 1-0-2, and back again.
tmux select-pane -t 0
tmux swap-pane -t 1
tmux swap-pane -t 1
# You can also swap panes by "marking" one and letting the target of the swap be
# implicit. Here, we change ordering from 0-1-2 to 1-0-2, and back again.
tmux select-pane -t 0
tmux select-pane -t 1 -m
tmux swap-pane
tmux swap-pane
Para mais, veja tmux (1).