When setting up an Anomaly it's useful to understand the sensitivity of the algorithm, in order to know how many alerts you are likely to receive. So in the preview you can see the last 3 months for Daily Time Period and the last 7 days for Hourly Time Period.
The Time Period field determines the data points that are used to determine Anomalies, therefore when selecting Hourly the algorithm will input each hour of the day to calculate the presence of an Anomaly. This is not to be confused with Frequency, which is how often the algorithm is run to check for the presence of Anomalies (Only available on higher plans).
NOTE: Not all Time Period, Metric and Segment combinations are possible and you will receive an error if the combination is invalid.
You have the option of customising the parameters used in the Anomaly Detection algorithm.