Sentence:  ΞA and ΞB are two <WSD>downtraces</WSD> ... from the two graphs. (Ξ<sub>A</sub> ∩ Ξ<sub>B</sub>) is the intersection between the two <WSD>downtraces</WSD>. 
Meaning of the word marked with <WSD> tags:  The subtree of a tree structure that is determined by an attribute of a parent node.