Atlassian's Jira is a favorite tool (especially) for software development teams because of its flexibility to accommodate various use cases. The APIs & underlying frameworks allow for easy extension of Jira features. There is a burgeoning ecosystem of apps (previously add-ons, plugins) owing to this extensibility.
Employee Success makes use of the APIs available, to integrate with Jira seamlessly. It can not only insert its own pages within Jira but also tightly couples with entities available in Jira such as projects, issues, users, sprints and many more. Framework for development is different for Server & Cloud add-ons, resulting in a few differences in behaviour of the two. These differences in some cases are purely technical in nature whereas in others have a functional impact.
The functional differences between the Server, Cloud & Data Center versions of the app are highlighted at appropriate places in this documentation. Another point worthy of a mention - is that Employee Success works seamlessly with Jira core, software as well as service desk.