@meejah Extras are additional sets of optional dependencies.
The idea here is that the package can include optional functionality without pulling in additional, heavy dependencies by default.
Let's say you're writing a telemetry library. It supports a handful of backends, including Datadog and New Relic, so it has code that expects the ddtrace and newrelic pip packages to be installed. But most users don't want to install all of the transitive dependencies for *both* of those backends.
So the package instead could define an extra for each backend. Someone can then install the package as `meejah-telemetry[newrelic]` and they get the New Relic dependencies, and when they go initialize it for NR functionality, it works. If they just install `meejah-telemetry` without the extra, and try to init it for NR, the library will complain about missing imports.