ディレクトリに「/」を追加すると、その意味はいつ変わりますか？

システム全体に適用されるルールはありません。ただし、表示される動作はrsyncrsync のマニュアルページの先頭に文書化されています。

          rsync -avz foo:src/bar /data/tmp

   This would recursively transfer all files from the directory src/bar on the machine foo into the /data/tmp/bar directory on the local machine. The files are transferred
   in  "archive" mode, which ensures that symbolic links, devices, attributes, permissions, ownerships, etc. are preserved in the transfer.  Additionally, compression will
   be used to reduce the size of data portions of the transfer.

          rsync -avz foo:src/bar/ /data/tmp

   A trailing slash on the source changes this behavior to avoid creating an additional directory level at the destination.  You can think of a trailing / on a  source  as
   meaning  "copy  the contents of this directory" as opposed to "copy the directory by name", but in both cases the attributes of the containing directory are transferred
   to the containing directory on the destination.  In other words, each of the following commands copies the files in  the  same  way,  including  their  setting  of  the
   attributes of /dest/foo: