What does [0], [1], ... in structure description means in X.509 RFC 5280?

Question

I'm studying RFC 5280 (https://www.rfc-editor.org/rfc/rfc5280) and have noticed strange marks ([0], [1], [2], [3]) in TBSCertificate structure definition

TBSCertificate  ::=  SEQUENCE  {
    version         [0]  EXPLICIT Version DEFAULT v1,
    serialNumber         CertificateSerialNumber,
    signature            AlgorithmIdentifier,
    issuer               Name,
    validity             Validity,
    subject              Name,
    subjectPublicKeyInfo SubjectPublicKeyInfo,
    issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                         -- If present, version MUST be v2 or v3
    subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                         -- If present, version MUST be v2 or v3
    extensions      [3]  EXPLICIT Extensions OPTIONAL
                         -- If present, version MUST be v3
    }

In ASN.1 it codes using special bytes 0xA0, 0xA1, 0xA2, 0xA3. Can't find any explanation on these bytes encoding. Can someone explain me?

Answer 1

It is a tagged type. See X.680 section 31.2. [0] means that the value is encoded with a context-specific class and number 0. In DER (X.690 sections 8.1.2 and 8.14), a constructed context-specific class with number 0 is encoded as 0xA0.