The Timeline is set up to display activities, which means only a limited selection of tables can be shown.

For custom tables, it is possible to display them in the Timeline by configuring them as activities.

More information on how to display a custom table in a Timeline can be found in the official documentation here: https://learn.microsoft.com/en-us/power-apps/maker/model-driven-apps/set-up-timeline-control#add-the-timeline-component-to-a-form

However, I've encountered a limitation that I find could significantly enhance the utility of the Timeline feature if addressed. Currently, there is no method to display information from entities such as Opportunities, Cases, on the Timeline.

Incorporating the ability to display such entities would greatly improve the convenience and informational value of the Timeline.