Not just qualify. You have to come through the group stage of Euro League or Champions League regulary.
Bayern, that is number four with 118 points reached the last 16 in 2010/11, the final in 2009/10 (lost against winner Inter), the last 8 in 2008/09 (lost against winner Barcelona), the last 4 in Uefa Europa League in 2007/08 (lost to winner St. Petersburg) and the last 8 in 2006/07 (lost against winner AC Milan).
The pots depend on the Uefa club coefficent.
http://en.wikipedia.org/wiki/UEFA_coefficient
Better take this link:
http://www.uefa.com/memberassociations/uefarankings/index.html
The 118 points mentioned above are from last years ranking that was important for this years draw.
*it is not only important how far you come but also how often you win or loose. You get points for every win and for every stage you conquer. Barcelona last year got 36 points as well as Barcelona - but I know that Barcelona had difficulties winning against little East European clubs in the group stage.
The ranking includes all clubs - so pot 1 can also mean a number 10 or 11 in the club rankings when some of the clubs only participate in Euro League.
This year Inter Milan (number 9) was the last in pot 9 with 100 points. AC (number 10) with 94 was in pot 2.
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Barcelona tied twice - once in Rubin Kazan and once in Copenhagen during group stage. The lost in London against Arsenal later on with one goal difference (won at home with 2) and tied at Madrid (but won at home by 2:0) - you see, Barcelona is beatable... ;)