What changes between rt-tokio and rt-tokio-current-thread?

[frederikhors]

I can read here:

For optimal performance, a batch exporter is recommended as the simple exporter will export each span synchronously on dropping. You can enable the [rt-tokio], [rt-tokio-current-thread] or [rt-async-std] features and specify a runtime on the pipeline builder to have a batch exporter configured for you automatically.

But what are these?

What changes between rt-tokio and rt-tokio-current-thread?