CIL is a levy that local authorities can choose to charge on new developments in their area. The money should be used to support development by funding infrastructure that the council, local community and neighbourhoods want.
CIL regulations have changed the developer payment landscape by introducing the levy and also by changing when councils can seek s106 obligations. Developer payments contribute to the provision of infrastructure or refurbishment of existing provision to support the additional burden new development makes on both local and strategic infrastructure.
If you are new to CIL and want to understand how it works we recommend that you read the Planning Practice Guidance on CIL: https://www.gov.uk/guidance/community-infrastructure-levy
This presentation aimed at Councillors may presentation also help you understand the relevance of viability and it will help you to understand the role of the Community Infrastructure Levy and other developer contributions: https://www.local.gov.uk/pas/find-event/event-presentations/councillor-briefings-section-3
You may find the information in the past PAS seminars about CIL useful in developing your approach More CIL Resources