At BA the commission equates to 10% of the total sales onboard. This 10% is then split by the number of people onboard (cabin crew only) PLUS one additional share for the CSD and each of the bar operators.
I.E. With 12 crew including one CSD and four bar operators you would have :
12
+ 1 CSD
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13 Shares
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+ 4 Operators
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17 Shares Ttl
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This way, everyone gets a share but those who manage the selling are rewarded with an additional slice of the comm!