Object Type is just a term used in software designating what is being integrated.
A Set Up is designed to sync Contacts, Leads, Accounts, Person Accounts ( under Contacts Object Type), and Candidates (These are called "Object Types") bi-directionally between your CRM and Marketing Automation, eCommerce, Financials, or Support software.
Depending on the integration like NetSuite, QuickBooks, Xero, Zendesk, and others you can also sync various Object Types like Invoices, Payments, Sales Orders, Cases, Tickets, and more... These actions are performed behind the scenes so there is nothing to select for Object Type however these are called "Object Types" too in software terminology.
Please note that a SyncApps subscriber can only select 1 Object type per Set Up. If wanting to sync another Object Type, please set up another Set Up.
* Candidates Object type is available on Bullhorn CRM only.