There isn't a huge difference between a covering letter and a query.... OK, I'm not going there!
Shika, I've looked it up and every definition of 'treatment' I can see in relation to writing is along the lines of:
"an outline of the characters, scenes and plot of a screenplay"
One of them said:
"a summary of a novel for use in a screenplay"
Or words to that effect. (I'm paraphrasing!)