Purpose
Because many messages cross a reader's desk (or computer screen) daily, the first thing she is
likely to ask upon seeing one is "Should I read this?" The "purpose" section answers that
question. A very direct opening like "The purpose of this memo is to ..." is perfectly acceptable.
It may be addressed to me.
Summary
The summary is a brief recounting of the entire memo, including discussion/background,
conclusions, and recommendation. Its placement as the second section allows a reader who
does not need to know the details to stop reading.