The sharing model defines how record-level privileges are granted to users who do not own the record. Configuring the sharing model is a two-part process. Organization-wide defaults are used to establish the most restrictive level of access for each object. Sharing reasons override the defaults to grant access to individual records.
Every object that allows record ownership has an organization-wide default setting dictating how records are shared between the owner and other users. Custom objects have several default settings:
Private—Records belong to the owner and only the owner. With the exception of ...