If a teacher is using the text response, both letters and numbers can be used to answer a question. If they use the number response, only the numbers are used to answer a question.
With an Ad Hoc question, the correct answer is not shown until after all the voting has stopped. The teacher can then go into the upper right corner where there is an image of AB and a check mark and choose the correct answer. This changes the correct answer to green (bar chart or pie chart) so that the teacher can quickly see if they need to re-teach that goal, or did the students really understand it.
Another tip that I learned is that once you start an Express Poll, if a student is putting in incorrect answers or silly answers, you can turn that voting device off at the board. That way the teacher doesn't waste time by talking to that particular student. I thought that was a very clever idea.
Image: 'Questions' http://www.flickr.com/photos/42788859@N00/318947873