The LogContext contains information required to trace distributed transactions or requests made to a distributed system. Such a trace may produce multiple log messages in one OPC UA Application and/or multiple log messages across multiple OPC UA Applications.

The TraceId is a unique identifier for a trace. Once assigned it never changes for that trace. If log messages for a given trace are across applications, they have the same TraceId.

A span represents a unit of work or operation. Spans are the building blocks of traces. A span is local to an OPC UA Application. The SpanId is a unique identifier assigned by an OPC UA Application for a local span in a trace. Spans can be nested. The ParentSpanId provides the reference to the parent span. The ParentSpanId is used to build a tree of spans inside a trace. If a nested span is created, the SpanId of the parent is used as ParentSpanId in the new child span.

At the very start (the root span) there is no ParentSpanId, only a new SpanId. This would be the base of a tree. See C.1 for additional discussion on SpanId and ParentSpanId.