Some works may only be available to certain audiences.
For example, a student may use a work from the classroom website or learning management system (Moodle). Likewise, an employee might cite resources from the company intranet when writing an internal report.
These types of sources are not available to the general public.
When the audience you are writing for can retrieve the resources you used (for example, a student writing a paper for an instructor can access their learning management system), cite the works using the normal format.
If, however, the work is for professional publication or intended for a wider audience who will not have access to these sources, cite the sources as personal communications.
See p. 259 in the manual for more information.
For an audience that can access the sources
(Newhall et al., 2017)
For professional publication or a wider audience that will not have access to the sources**
(S. J. Greene, personal communication, March 3, 2015)
**Note: Personal communications do not have a reference entry and are only cited in the text of a paper.