I am a nonmedical scientist trying to make sense of medical terms that were involved in the following study: Transmission onset distribution of COVID-19. I am attaching a link to the paper for your reference: https://www.sciencedirect.com/science/article/pii/S1201971220306123
My question is: what is a layman's definition of what transmission onset is? Can I construed it as the infectiousness of the disease or transmissibility of the disease?
The figure presents the probability density function vs number of days after onset of symptoms. The caption reads: Estimated distribution of the transmission onset of COVID-19 relative to the onset of symptoms.
Can someone kindly explain this 'transmission onset' thing in plain language please?