Clarify flags meaning

[bogdandrutu]

Per definition of Span.flags and Link.flag which are the same, I am confused what value to use and where.

For Span:

  // Flags, a bit field.
  //
  // Bits 0-7 (8 least significant bits) are the trace flags as defined in W3C Trace
  // Context specification. To read the 8-bit W3C trace flag, use
  // `flags & SPAN_FLAGS_TRACE_FLAGS_MASK`.
  //
  // See https://www.w3.org/TR/trace-context-2/#trace-flags for the flag definitions.
  //
  // Bits 8 and 9 represent the 3 states of whether a span's parent
  // is remote. The states are (unknown, is not remote, is remote).
  // To read whether the value is known, use `(flags & SPAN_FLAGS_CONTEXT_HAS_IS_REMOTE_MASK) != 0`.
  // To read whether the span is remote, use `(flags & SPAN_FLAGS_CONTEXT_IS_REMOTE_MASK) != 0`.
  //
  // When creating span messages, if the message is logically forwarded from another source
  // with an equivalent flags fields (i.e., usually another OTLP span message), the field SHOULD
  // be copied as-is. If creating from a source that does not have an equivalent flags field
  // (such as a runtime representation of an OpenTelemetry span), the high 22 bits MUST
  // be set to zero.
  // Readers MUST NOT assume that bits 10-31 (22 most significant bits) will be zero.

For Link:

    // Flags, a bit field.
    //
    // Bits 0-7 (8 least significant bits) are the trace flags as defined in W3C Trace
    // Context specification. To read the 8-bit W3C trace flag, use
    // `flags & SPAN_FLAGS_TRACE_FLAGS_MASK`.
    //
    // See https://www.w3.org/TR/trace-context-2/#trace-flags for the flag definitions.
    //
    // Bits 8 and 9 represent the 3 states of whether the link is remote.
    // The states are (unknown, is not remote, is remote).
    // To read whether the value is known, use `(flags & SPAN_FLAGS_CONTEXT_HAS_IS_REMOTE_MASK) != 0`.
    // To read whether the link is remote, use `(flags & SPAN_FLAGS_CONTEXT_IS_REMOTE_MASK) != 0`.
    //
    // Readers MUST NOT assume that bits 10-31 (22 most significant bits) will be zero.
    // When creating new spans, bits 10-31 (most-significant 22-bits) MUST be zero.
    //
    // [Optional].

Is it true that for the Span.flag we expect the Bits 0-7 to be the TraceFlags of the current Span (not the parent), but the remote property is for the parent. For the Link.flag we expect the Bits 0-7 to be the TraceFlags of the linked SpanContext as well as the remote property. The confusion I have is that where do we have the Span parent's SpanContext TraceFlags? If they are important for links why not having them for parent (which is in some way a more special link)?