Hi Hoat Le
MAny people have been posting the same question.kindly go through the posts on buffer selector that have already been discussed.
You will find the answer for your query.
For your convenience i repost my answer again.
buffer selector finds its use in systems which use multiple system iterations.Its significance is to filter out a particular iteration and then only the signal of that particular iteration is forwarded to the detector.
Make sure if you use a buffer selector in your design,,you change the signal index in the analyser window to the same iteration value as you set for your buffer selector.Or else you will find no output values for BER and Quality factor at all other signal index values.
Hope it helps
Regards