In digest comparisons to previous periods are used.
So for example if the digest is for the previous day i.e. January 2nd.
Then the data from this range will be compared with the data from the day before January 1st.
You will then be able to see a percentage increase/decrease of sales etc.
Please note that when a digest is sent the current day is not included, so if a digest is sent at 11am then any sales made on that day will not be included until the next digest.