What is the Btrfs extended attribute 'm'?

cn flag

I recently found one of my Virtual Machine images with the extended attribute 'm'. I tried to look it up, but I found no information. Is this an undocumented extended attribute?

$ lsattr usb2ubuntu.btrfs.img*
---------------------m usb2ubuntu.btrfs.img
---------------------- usb2ubuntu.btrfs.img.210810_15:02:49.bak

Notice the attribute doesn't exist on the backup file.

vn flag
Add the output of `lsattr -l` (long names instead abbreviations)
us flag

The chattr manpage says:

   m      A file with the 'm' attribute is excluded from compression
          on file systems that support per-file compression.

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