What's that % to the rightmost part of a dataset name?

Can you, please, propose a head-start on reading about the % to the rightmost position of a dataset name?

For instance:

$ zfs list -r -t all ~
NAME                            USED  AVAIL  REFER  MOUNTPOINT
rpool/export/home/user2         902K  1023M   870K  /export/home/user2
rpool/export/home/user2@backup     0      -   870K  -
rpool/export/home/user2%           -      -      -  /export/home/user2
rpool/export/home/user2/test     31K   100M    31K  /export/home/user2/test
rpool/export/home/user2/test%      -      -      -  /export/home/user2/test

Thanks in advance.