What do the symbols in the left-most column of ncdu mean?

Question

OS: Kubuntu 16.04 and ncdu version 1.11.

When I run

ncdu -x /

I get

--- / --------------------------------------
.  97.1 GiB [##########] /home              
    4.6 GiB [          ] /usr
  718.8 MiB [          ] /lib
. 562.4 MiB [          ] /var
  177.8 MiB [          ] /opt
  143.2 MiB [          ] /boot
   12.7 MiB [          ] /bin
.  11.1 MiB [          ] /etc
   11.0 MiB [          ] /sbin
. 100.0 KiB [          ] /tmp
   36.0 KiB [          ] /mnt
.  16.0 KiB [          ] /media
!  16.0 KiB [          ] /lost+found
    4.0 KiB [          ] /lib64
e   4.0 KiB [          ] /srv
!   4.0 KiB [          ] /root
e   4.0 KiB [          ] /cdrom
@   0.0   B [          ]  initrd.img.old
@   0.0   B [          ]  initrd.img
@   0.0   B [          ]  vmlinuz.old
@   0.0   B [          ]  vmlinuz
>   0.0   B [          ] /sys
>   0.0   B [          ] /run
>   0.0   B [          ] /proc
>   0.0   B [          ] /dev

What do ., !, e, @, and > mean? man ncdu and info ncdu don't have any information on the meaning of these symbols.

man du doesn't help either.

Answer 1

While in ncdu, press ? to show help, then 2 to show the formatting help.

Here it is :

┌───ncdu help─────────────────1:Keys───2:Format───3:About──┐
│                                                          │
│  X  [size] [graph] [file or directory]                   │
│   The X is only present in the following cases:          │
│                                                          │
│   !  An error occured while reading this directory       │
│   .  An error occured while reading a subdirectory       │
│   <  File or directory is excluded from the statistics   │
│   >  Directory was on an other filesystem                │
│   @  This is not a file nor a dir (symlink, socket, ...) │
│   H  Same file was already counted (hard link)           │
│   e  Empty directory                                     │
│                                                          │
│                                         Press q to close │
└──────────────────────────────────────────────────────────┘