You need this for rclone mount in the docker container to be visible on the host.
The rclone mount is a sub mount, because the first mount is mounting /data into the docker container's file system. That is known as a bind mount to glue on a bit of filesystem somewhere else - hence the "bind propagation". The submount is the rclone mount mounting on to /data
Here are the docs which are pretty impenetrable!
shared Sub-mounts of the original mount are exposed to replica mounts, and sub-mounts of replica mounts are also propagated to the original mount.
It would help if they explained what a "replica mount" is (I don't know!).
There is a stack overflow answer here which is quite helpful