The only threat to her comes, paradoxically, from within her own ranks. Since the second world war, the centre-right camp in German politics has had a peculiar arrangement: the Christian Democrats appear on ballots all over Germany except in Bavaria, where a “sister party”, the Christian Social Union (CSU), takes the CDU’s place. The two are independent but by convention campaign together in federal elections and form one group in the Bundestag.
http://dictionary.cambridge.org/dictionary/english/convention
I found 2 main definitions:
[ C ] a large formal meeting of people who do a particular job or have a similar interest, or a large meeting for a political party:
[ C or U ] a usual or accepted way of behaving, especially in social situations, often following an old way of thinking or a custom in one particular society:
Which one does it fit better in this case?
Source: http://www.theworldin.com/edition/2017/article/12596/permanent-chancellor
Best Answer
The latter definition is the correct one in this case. In the context of the sample text, it means that the CSU and CDU are technically independent parties, but they almost always campaign together and come together as one in the Bundestag.