A Field is a basic unit of information (ex: ID, last name, address, etc.) that is specific to a Business Object (ex: Incident, Problem, Service, Customer, etc.). Field options only appear in drop-downs when you are creating a Comparison Clause.
CSM provides you with multiple Field options, including the Field of the current Business Object (ex: Incident) along with the Fields from related Business Objects (ex: Change Request, Customer, etc).
If you have security rights, you can use the Field Properties window to set Field properties such as data type, size, read-only, required, and default value.