A question about how to check a rewriting system is confluent?

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I am reading the book "Computation with finitely presented groups" on page 59, enter image description here

I don't know why we need only test six words? does any other words can be generated by the six words? it is seems not.

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Do you understand the definition of local confluence? You are looking for pairs of left hand sides of rules of the form $uv$ and $vw$ for strings $u,v,w$. Then, to test local confluence, you reduce the word $uvw$ in two different ways and check that the results are the same.

The six listed words are the only possible words $uvw$ arising from the three left hand sides in the rewriting system. For example, the first one $xyzx$ comes from the left hand sides $xyz$ and $yzx$ with $u=x$, $v=yz$, $w=x$.