This read-only property contains issuer unique identifier.
This unique identifier can be used when it's necessary to distinguish issuers in case when X.500 name has been used for different objects.
The following paragraph is taken from RFC 2459 (Housley, et. al.), part 126.96.36.199 Unique Identifiers:
«These fields may only appear if the version is 2 or 3 (see sec. 188.8.131.52). The subject and issuer unique identifiers are present in the certificate to handle the possibility of reuse of subject and/or issuer names over time. This profile recommends that names not be reused for different entities and that Internet certificates not make use of unique identifiers. CAs conforming to this profile SHOULD NOT generate certificates with unique identifiers. Applications conforming to this profile SHOULD be capable of parsing unique identifiers and making comparisons.»