The Christians at Eßlingen were very interested in Zwingli’s Reformed theology but they had questions, since it overturned life-long held beliefs. So Zwingli wrote two letters of explanation, the second sent on the 16th of October, 1526. In it he expresses appreciation for them and admiration for their struggle to understand.
The particular difficulty was, of course, Zwingli’s idea about the Supper. So he explained very briefly his concept. Apparently it was all they needed to know, because a third letter of explanation was never asked for.