This holds the actual key or certificate data (or a fragment of it) associated with one stored fragment of a certificate/key used to secure the connection to the external VMM. This is the sensitive payload UCS Manager uses to authenticate or secure its connection related to that key fragment, which is exactly why it's read-only over SNMP rather than something an admin is meant to extract this way. In practice this field would contain PEM-encoded certificate text rather than a short human-readable value, and its exact content matters only to the internal crypto library consuming it.