When defining local tasks (= tabs) in your Drupal 8 modules, you can specify a title for the tab via the 'title' property in your [MODULENAME].links.task.yml file.
However, in some cases you might want to make the task title dynamic, e.g. depending on the context of the entity where the tab is displayed.
This can be achieved by overriding the \Drupal\Core\Menu\LocalTaskDefault class with your own class for that tab.
Here is an example that uses a callback function to dynamically set the title, both for the route and the local task: