This is a server side feature that allows automatic updates of a superproject repository when when a change is merged on tracked submodules.  The subscription configuration done at the submodule detailing how to update the superproject.  This allows for superprojects to get automatic updates from all, some or none of the submodules associated with them.

This is a feature we are currently using in Googles Gerrit product.  https://gerrit-review.googlesource.com/Documentation/user-submodules.html#_creating_a_new_subscription

We are expected to migrate software projects using this feature in Gerrit to BitBucket.  This migration will be hindered if Bitbucket does not support these subscriptions.  We have considered using an external client to Bitbucket to make these subscription updates. However, anything done external to the Bitbucket server will suffer from potential update timing issues.