Why high and lower bit of generator must be 1?


Here is an excerpt from Andrew S. Tanenbaum, Computer Networks, 5th edition, Chapter 3 (The data link layer), Page 213:

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[Both the high and low order bit of the generator must be 1.]

My question is how do you determine high order bit? And why high order bit and lower order bit, both must be one? From my understanding, it is used to detect burst error but is my understanding true?