There are two types of objects in Salesforce
Standard Objects are objects provided by Salesforce.com and include objects such as accounts, contacts, opportunities, leads, products, users, contracts, reports, and the dashboard.
CLM supports standard objects which can be mapped to SFDC at the Account, Contact, Contract Opportunity, Quote, and Quote Line Item entity.
See URLs listed below for additional information on standard opportunity, contract, contact, and account fields in SFDC:
Opportunity Fields: https://help.salesforce.com/articleView?id=opp_fields.htm&type=0
Contract Fields: https://help.salesforce.com/articleView?id=contract_fields.htm&language=en&type=0
Contact Fields: https://help.salesforce.com/articleView?id=contacts_fields.htm&type=0
Account Fields: https://help.salesforce.com/articleView?id=account_fields.htm&language=en&type=0
Custom objects store information that is unique to your organization such as, custom fields, page layouts, relationship to other objects in SFDC, and the custom user interface tab.
Note: The SFDC-CLM integration can be used to map standard objects with custom fields created in SFDC. However, it cannot be used to map custom objects with custom fields.