categorical assessment that location identifiers never “concern[] the substance, purport, or meaning” of a communication misses the mark.35 Often, a location identifier serves no routing function, but instead comprises part of a communication’s substance.36 As a leading treatise on criminal procedure explains: [T]he line between content and non-content information is inherently relative. If A sends a letter to B, asking him to deliver a package to C at a particular address, the contents of that letter are contents from A to B but mere non-content addressing information with respect to the delivery of the package to C. In the case of email, for example, a list of e-mail addresses sent as an attachment to an e-mail communication from one person to another are contents rather than addressing information. In short, whether an e-mail address is content or non-content information depends entirely on the 37 circumstances. In essence, addresses, phone numbers, and URLs may be dialing, routing, addressing, or signaling information, but only when they are performing such a function. If an address, 35 18 U.S.C. § 2510(8). 36 See generally Orin Kerr, Websurfing and the Wiretap Act, Wash. Post. (June 4, 2015), https://www.washingtonpost.com/news/volokhconspiracy/wp/2015/06/04/websurfing-and-the-wiretap-act/. 37 Wayne R. LaFave, et al., 2 Crim. Proc. § 4.4(d) (3d ed.). 20