Topic: The AWR-report

All hello!
Question on interpretation of usage PGA of the differential AWR-report.
Explain please as in such report the digit is formed
max PGA allocation carried to DB time, to Elapsed time (a difference that the first parameter shows growth PGA from direct path read...?.)?
a maximum of selection PGA of lifetime of session?
These are the data on sessions, which at creation , restricting the report,
The accumulative statistics was active or is?
I thank