It has a central so called "REST API" that is deployed on your server and is serving all changes directly to your clients. This lowers traffic and makes it robust.
This is the main client for LegionBoard and always supports every feature of Heart.
Thanks to Vertretungsplan.me the changes plan of LegionBoard can be displayed nicely even on mobile devices and the users get notified on changes.
You can always look at the code of LegionBoard. It is open source with repositories on GitLab and GitHub.
LegionBoard is based on the newest technologies. Many big companies like Google are using the same stuff.
All the changes can be viewed on any device with low traffic usage.
You can get the source code and deploy it yourself. There are no costs for you. Promised.
Compared to other solutions like PDF schedules that have to be downloaded each time, LegionBoard is very resource efficient and doesn't need much traffic.
LegionBoard was developed with security in mind. Thanks to being open source, this can be reviewed every time.