We have the following requirement:
We have a custom field named "Customer". It is not required to fill out that field.

Now if this field is filled out and someone is changing the issue field "version" to another version or is changing the remaining estimation to another value then JIRA should notify the user that there was a change.

This is important because we have internal and external requirements. If it is a external requirement of a customer and we are changing the estimation (it means: we need more time or we are faster) or if we switch the requirement to a later or earlier version then the worker at the ticket should be notified if the field "customer" is filled out. The reason is that the worker can sent the customer a quick feedback that something on his requirement has changed in estimation or version.