No. If you don't know the source of law, please leave the field blank.
If you know the source of law that would be appropriate, go ahead and
include it. Often, a non-lawsuit threat such as an email or letter
includes specific legal claims and citations to statutes, which may
help you identify the source of law for that threat. This most likely would require that you have some background in the law.
If known, list whether U.S. federal law, an individual state's law, or international laws apply to the situation or lawsuit. This usually requires looking at the sending/suing party's legal claims and noting into what category they fit. You can also consider the source of any defenses that the receiving/defending party has asserted. Note that some cases may include multiple sources of law, particularly if there are claims related to both state and federal law.
The source of law may be noted in related court documents, though it may be difficult to determine if you aren't familiar with the law.
