This question was origined from an experience of conferencing with different meeting terminals, including Polycom, Cisco, Tandburg, Huawei, etc.

In our current implementation of SIP conference, we are using a stream_id tag in the Video Fast Update command to  tell the peer we are requesting an Intra frame for a specific stream. And the stream_id tag value was recorded from the Label attribute of the SDP exchange process.

However, this situation was:
1. Some of the vendors don’t have stream_id in the VideoFastUpdate command, such as CISCO and Tandburg, but if we send a VideoFastUpdate with stream_id tag in it, it doesn’t matter, it can response a 200 OK, only the stream_id value can not be zero, otherwise, it will reply with a 500 error.
2. Polycom does have a stream_id in it, but no matter what circumstances, the stream_id is alway 1.
3. Huawei seems have a same implementation with Kedacom, having a stream_id in it, and the value is coherence with the LABEL tag in the SDP.

Then I turned to the RFC document, RFC5168: XML Schema for Media Control, category: informational, developed by Microsoft, Polycom, Radvision.
The definition is placed in phase 5 of this document:
The Schema Definition

  <?xml version="1.0" encoding="utf-8" ?>

   <xs:schema id="TightMediaControl"
    elementFormDefault="qualified"
     xmlns:xs="http://www.w3.org/2001/XMLSchema">

           <xs:element name="media_control">
               <xs:complexType>
                  <xs:sequence>
                     <xs:element name="vc_primitive"
                                           type="vc_primitive"
                                           minOccurs="0"
                                           maxOccurs="unbounded" />
                     <xs:element name="general_error"
                                           type="xs:string"
                                           minOccurs="0"
                                           maxOccurs="unbounded" />
                  </xs:sequence>
               </xs:complexType>
           </xs:element>

           <!-- Video control primitive.  -->

           <xs:complexType name="vc_primitive">
                   <xs:sequence>
                     <xs:element name="to_encoder" type="to_encoder" />
                      <xs:element name="stream_id"
                                       type="xs:string"
                                       minOccurs="0"
                                       maxOccurs="unbounded" />
                           </xs:sequence>
           </xs:complexType>

           <!-- Encoder Command:
                Picture Fast Update
           -->

           <xs:complexType name="to_encoder">
                   <xs:choice>
                           <xs:element name="picture_fast_update"/>
                   </xs:choice>
           </xs:complexType>

   </xs:schema>

So, as you can see, there is actually a stream_id tag in it. But when I tried to find more about it, nothing was found. Weird enough for a RFC document.

After re-read the full document, found out there was a description which explains the situation:
New implementations are discouraged from using the method described except for backward compatibility purposes. New implementations are required to use the new Full Intra Request command in the RTP Control Protocol (RTCP) channel.