Permissions planning for sites and content in SharePoint Server
APPLIES TO: 
2013 2016 2019 Subscription Edition 
SharePoint in Microsoft 365
Some sites in an enterprise probably contain content that should not be available to all users. For example, proprietary technical information should be accessible only on a need-to-know basis. An intranet portal for employee benefits should be available only to full-time employees, whereas the home page of an Internet Web site is accessible by anonymous clients.
Permissions control access to sites and site content. You can manage permissions by using SharePoint groups, which control membership. Fine-grained permissions also help to secure content at the item and document level.