select="$sections/section[@group=1]/item[@id=1][$pos]"/>

the $pos item node that is a child of section[@group=1]
and has @id=1.

Since you have uniue ids within each section, if $pos is 1
you should get the same as 


select="$sections/section[@group=1]/item[@id=1]"/>

but id pos is any number other than 1 you should get nothing as 
there is only one item in each step satisfying [@id=1] so position() is
always equal to 1.

David


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