In case a file is formatted as PEM, each called file handler receives the PEM name (everything following any '"-----BEGIN "') as well as possible PEM headers, together with the decoded PEM body. Since PEM formatted files can contain more than one object, the file handlers are called upon for each such object.
If the file isn't determined to be formatted as PEM, the content is loaded in raw form in its entirety and passed to the available file handlers as is, with no PEM name or headers.
Each file handler is expected to handle PEM and non-PEM content as appropriate. Some may refuse non-PEM content for the sake of determinism (for example, there are keys out in the wild that are represented as an ASN.1 OCTET STRING. In raw form, it's not easily possible to distinguish those from any other data coming as an ASN.1 OCTET STRING, so such keys would naturally be accepted as PEM files only).
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.