At one time we proposed that a streamed node should carry with it some kind of set of counters indicating basic information about the number of preceding siblings of different kinds. I don't believe there is now any functionality in the spec that relies on this. However, I found a couple of references to the idea:

Section 2.10 "However, constructs (for example, simple forms of xsl:number, and 
simple positional patterns) that require knowledge of the number of preceding elements by name are permitted."

Similarly in 4.6: "Summary data about the preceding siblings of the node, and of each of its ancestor nodes: 
specifically, for each distinct combination of node kind, node name, and type annotation, a count of the number 
of preceding siblings that have that combination of properties. This information allows patterns such as 
match="para[1]" to be used, and it permits some limited use of the xsl:number instruction."

I think these two paragraphs are obsolete and should be deleted.

(For info: Saxon's streaming implementation does in fact carry this information, but makes very little use of it; about the only use I can find is to support simple positional patterns such as para[1]).