Which among the following statement is TRUE about 'serial interval'?

The term 'serial interval' is applied in relation to outbreaks and describes 'the gap in time between the onset of primary case and secondary case'. It gives an indication of the incubation period during an outbreak of the disease. Incubation period is the time interval from infection to the onset of clinical symptoms. Generation time is interval of time between receipt of infection by a host and maximal infectivity of that host. The communicable period, or period of infectivity, is the time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to humans, or from an infected person to animals.