I am aware that you can stop at either one of the three dots, but not both. However, sometimes I hear reciters not stop at all, is it compulsory to stop at either one of the three dots?
This stop represents a point where there are two acceptable (but mutually exclusive) ways to read a particular line. In this particular example,
- This is the book without doubt, in it is guidance for those who fear God.
- This is the book; there is no doubt in it, a guidance for those who fear God.
To my understanding (see also http://www.ilmfruits.com/2008/tajweed-different-stops/), the stop itself is weighted the same as a jeem stop (ج), meaning it's a place you can stop if you want, but there's no particular recommendation one way or the other. However, if you do stop at one you must not stop at the other otherwise the meaning is changed entirely. So, for example, if you stopped at both points the line would be parsed thus:
- This is the book, no doubt. In it. Guidance to those who fear God.