without a medical diagnosis .for PTSD at the time of the previous claim.” Id. at 19.
III.
For the following reasons, we hold that the Veterans Court did not misinterpret § 3.156(c)(1). We therefore affirm the court’s decision affirming the Board’s decision denying Mr. Kisor entitlement to an effective date earlier than June 5,2006, for the grant of service connection for PTSD.
At the heart of this appeal is Mr. Kisor’s challenge to the VA’s interpretation of the term “relevant” in 38 C.F.R. § 3.156(c)(1).10 As a general rule, we defer to an agency’s interpretation of its own regulation “as long as the regulation is ambiguous and the agency’s interpretation is neither plainly erroneous nor inconsistent with the regulation.” Gose v. U.S. Postal Serv., 451 F.3d 831, 836 (Fed. Cir. 2006) (citing Gonzales v. Oregon, 546 U.S. 243, 126 S.Ct. 904, 163 L.Ed.2d 748 (2006); Christensen v. Harris Cty., 529 U.S. 576, 588, 120 S.Ct. 1655, 146 L.Ed.2d 621 (2000); Bowles v. Seminole Rock & Sand Co., 325. U.S. 410, 413-14, 65 S.Ct. 1215, 89 L.Ed. 1700 (1945)); see also Long Island Care at Home, Ltd. v. Coke, 551 U.S. 158, 171, 127 S.Ct. 2339, 168 L.Ed.2d 54 (2007) (“[A]n agency’s interpretation of its own regulations is controlling unless plainly erroneous or inconsistent with the regulations being interpreted.” (internal quotation marks omitted) (quoting Auer v. Robbins, 519 U.S. 452, 461, 117 S.Ct. 905, 137 L.Ed.2d 79 (1997))).
We hold that § 3.156(c)(1) is ambiguous as to the meaning of the term “relevant.” In our view, the regulation is vague as to the scope of the word, and canons, of construction do not reveal its meaning. See Gose, 451 F.3d at 839 (ruling that a regulatory phrase is ambiguous when “the regulation is vague as to the scope of the phrase”); Cathedral Candle Co. v. Int’l Trade Comm’n, 400 F.3d 1352, 1362 (Fed. Cir. 2005) (holding a statute ambiguous when “traditional tools • of statutory construction” did not resolve the construction dispute). Significantly, § 3.156(c)(1) does not specify whether “relevant” records are those casting doubt on the agency’s prior rating decision, those relating to the veteran’s claim more broadly, or some other standard. This uncertainty in application suggests that the regulation is ambiguous. See, e.g., Abbott Labs. v. United States, 673 F.3d 1327, 1331 (Fed. Cir. 2009) (holding the regulatory term “affect” was ambiguous when the regulation did not specify the types of effects falling within its scope).
The varying, alternative definitions of the word “relevant” offered by the parties further underscore § 3.156(c)(l)’s ambiguity. See Nat’l R.R. Passenger Corp. v. Bos. & Me. Corp., 503 U.S. 407, 418, 112 S.Ct. 1394, 118 L.Ed.2d 52 (1992) (“The existence of alternative dictionary definitions ... , each making some sense under the statute, itself indicates that the statute is open to interpretation.”); Hymas v. United States, 810 F.3d 1312, 1320-21 (Fed. Cir. 2016). In his briefs; Mr. Kisor defines “relevant” in a way mirroring the federal rules of evidence. Compare Appellant’s Br. 9-10 (defining “relevant” as “any tendency to make the existence of any fact that is of consequence to the determination of the action more [or less] probable” (emphasis omitted)), with Fed. R. Evid. 401(a)-(b) (defining “relevant” as “any tendency to make a fact more or less probable” when the “fact is of consequence in determining
10
The Board interpreted 38 C.F.R. § 3.156(c)(1) when it ruled that Mr, Kisor’s service department records were not “relevant” under that subsection. See J.A. 90-91. Because the Board is part of the VA, see 38 U.S.C. § 7101(a); Henderson ex rel. Henderson v. Shinseki, 562 U.S. 428, 431, 131 S.Ct. 1197, 179 L.Ed.2d 159 (2011), the Board’s interpretation of the regulation is deemed to be the agency’s interpretation.