Final answer:
A file memorandum should primarily include primary legal sources, but secondary sources might be used for context, unless guidelines specifically advise against it. Personal communications are excluded since they are not publicly accessible for research.
Step-by-step explanation:
The question pertains to the inclusion of citations to secondary sources of tax law in a file memorandum. It is commonly advised that a file memorandum should focus on the primary legal sources such as statutes, regulations, and case law. However, there might be scenarios where referencing secondary sources like commercial tax services can be beneficial for explanatory purposes or to provide context, especially when the primary sources are not entirely clear. That said, if the specific instructions for the memorandum advise against the inclusion of secondary sources, they should be omitted. This is often the case because personal communications like emails, interviews, and other unverifiable sources lack the public accessibility required for broader research.