; (2) in the case of a claim regarding a disability incurred or aggravated during service in the active military, naval, air, or space service, the assertion of former member of the Armed Forces regarding the circumstances surrounding its incurrence or aggravation; (3) a credible buddy statement; or (4) such other sources of evidence or processes as the Secretary of Veterans Affairs deems appropriate for purposes of determining eligibility or entitlement under subsection (a).