A position in a distributed trace, in W3C Trace Context terms (design 4.58): the trace (traceId, 32 lowercase hex digits), one span in it (spanId, 16 lowercase hex digits) and whether the trace is sampled. Neither id is all zeros. It is the serializable carrier a caller stores or sends so that work done elsewhere - another thread, another process, another machine - continues the same trace: written as a traceparent header value (traceParent, 00-<traceId>-<spanId>-<flags>) and read back with TraceContext.parse; its JSON form is that string.
Whoever continues a context makes it the remote parent of the span it opens, so that span keeps the context's trace id. A backend that cannot express a parent ignores it.
The context of the span key names in this trace: the same trace id and sampling, and a span id derived from the trace id and key alone (the first 8 bytes of a SHA-256), so every process that names the same span the same way computes the same id, with no coordination. The new span's parent is whatever its emitter says; it need not be this context's span.
The context of the span key names in this trace: the same trace id and sampling, and a span id derived from the trace id and key alone (the first 8 bytes of a SHA-256), so every process that names the same span the same way computes the same id, with no coordination. The new span's parent is whatever its emitter says; it need not be this context's span.