something is a special kind of mark; you can use it to mark messages for
‘something’, and then decide later what the ‘something’ should be14. Later, you can set the actual mark using
M-x mu4e-mark-resolve-deferred-marks (#). Alternatively, mu4e
will ask you when you try to execute the marks (x).
After marking a message, the left-most columns in the headers view indicate
the kind of mark. This is informative, but if you mark many (say, thousands)
messages, this slows things down significantly15. For this reason, you can disable this by setting
mu4e-headers-show-target to nil.
This
kind of ‘deferred marking’ is similar to the facility in dired, midnight
commander (https://www.midnight-commander.org/) and the like, and uses
the same key binding (insert).
this uses an Emacs feature called overlays, which are slow when used a lot in a buffer