What counts as "relevant" evidence the plan must give you
The claim-file rule is not limited to what the plan actually relied on. Here is the regulation's own four-part definition of what counts as relevant.
You are entitled to request your complete claim file free of charge, but the regulation is specific about what counts as part of that file. Under 29 CFR 2560.503-1(m)(8), a document, record or other information is relevant to your claim if it meets any one of four tests, and the narrowest of them, what the plan actually relied on, is only the first.
The first test is the obvious one: material the plan actually relied on to decide your claim. The second is broader and often overlooked: material that was submitted, considered, or generated during the claim decision, whether or not the plan ended up relying on it. That second category is what makes an internal reviewer's note, a vocational consultant's file, or a peer-review report you were never shown part of what you can request, even if the denial letter never mentions it.
The regulation adds two further tests: material demonstrating compliance with "the administrative processes and safeguards required pursuant to paragraph (b)(5) of this section", which is the regulation's own safeguard requirement rather than whatever process the plan wrote for itself, and, for disability and group health claims specifically, a statement-of-policy or guidance test the regulation also lists. Practically, a request for your complete claim file should ask for everything considered or generated in connection with the claim decision, not only the documents the denial letter cites, since the broader category is what the regulation actually promises.
Why this matters for your appeal, not only for curiosity
Material the plan considered and did not rely on can be exactly what shows a decision was inconsistent, incomplete, or based on a partial reading of your medical record. Requesting the full file before you write your appeal, not after, gives you a chance to respond to everything actually in front of the plan, which matters given how courts reviewing a denied claim generally limit themselves to the record built during the internal appeal.