The Writing tab is part of the Component Editor. It is the assignment page for all text associated with a particular page of a course. The Writing tab differs for each template and contains various combinations of elements. For details on the elements that you can add to the various templates, see the desired template types or view the individual element descriptions in the section on Working with Elements.
See the Writing Tab manual for detailed information on how to use the writing tab elements in conjunction with the various template types to build instructionally sound course components.