What are 'function' field types in Quotes/Contracts Modules?

In Appendix A of the SuiteCRM USer Guide, in the Quotes and Contracts Field List (Appendix A :: SuiteCRM Documentation) there is a Field Type ‘function’ listed for Line Items. When I am in my admin Studio ->Quotes->Fields->Add Field in drop down menu ‘Data Type’ I do not have an option for ‘function’ as a field.

What is a ‘function’ field? How can it be used generally? Where can I learn more about how to work with adding a function as a field in Quotes and relating it to the Line Items in the Quotes module?

Aldo I’m not 100% sure about it, I can assume the type function is not an actual data type but a full functionality behind that field(or group of field as is in the case of Line Items).
In fact, Line Items is a full separate module in it self. When you work on Quotes, Invoices, Contracts… or any module that uses Line Items nativily, the entire “Line items” component is just a reference to the actual module.

Hope it helps to clarify that for you.