What do the dashed lines separate in the debug navigator?

Looking at:

I can't understand why are things grouped together between dashed lines.

_{I cropped the screenshot to remove method named I didn't want to share. But this screenshot is entirely from Thread 1 Queue : com.apple.main-thread (serial)}

I already thought of the following three but all seem to be incorrect.

• It's separated by classes. Not true: different classes were on the same side of dashed lines.
• It's separated by directories. Not true: files that were both in RxSwift directory were on different side of a dashed line.
• It's separated by modules. Not true: files from a pod and my own code were on the same side of a dashed line.

I also looked at: Examine threads and stacks in the Debug navigator but didn't find information on how they're grouped.

There are lines here that are collapsed (look at the numbers) new messages

You can click a button down in the filter field at the bottom to have it expand the missing lines in the trace