Feedback: summary fields are a fantastic feature for consolidating reports on the same view as the detail. It is challenging at the moment to document the purpose and intent of each summary field, unless one adds the description to the summary field name.
Idea: Add summary field descriptions (similar to columns) that can be seen when the user's mouse hovers over the field. Alternatively (or additionally), a description label could be included underneath the summary field. This would help sheet creators and users to recall the purpose of the summary field.