What does the grey line mean in Studio?

In Studio, the grey line to the left of a paragraph indicates the audio generation status. A light grey line signifies that audio has not yet been generated, while a dark grey line indicates that audio has been successfully generated.

In the image below, the highlighted paragraph has audio generated, whereas the second one does not.

In Dark Mode, this color scheme adjusts slightly: a light grey line still represents ungenerated audio, while a white line indicates that the paragraph has been converted to audio.

The image below demonstrates this: audio has been generated for the highlighted paragraph, but not for the second one.