Example a can be termed a face threatening act as it implies that the recipient of the instructions in stupid, unable to interpret the catalogue correctly. Such a formulation will be avoided by a writer of a business letter. On the other hand example b can be seen as a face saving act, as the writer assumes responsibility for the error, thus avoiding to offend the other party. Such a strategy, which is as act of politeness, is a way of avoiding conflict and of continuing business relations.