Whether you conduct your research at a research site or PIC will affect the application route.
A research site is the single organisation responsible for conducting the research at a particular location. The research site is not necessarily the location where research activities will actually take place.
For example, in a research project by practice nurses from GP practices, interviews with participants may take place in the participant's home, but the research site would be the GP practice.
A Patient Identification Centre is as an organisation which refers potential participants to the research team for assessment and possible recruitment.
For example, a GP may identify potential participants from their practice database, provide patients with information about a study and seek consent to pass on the patients' contact details to the research team.