Fortigate 90E clear session vs drop packet

Hello

I am trying to familiarize myself with fortigate router.  At times, it can be confusing - for example the IPs DoS policy is set to "block"  however if I check Fortiview/traffic from WAN/Threats (when I receive notification of a UDP flood condition)

then I can view sessions with action "clear session" while above that in the "summary of" area/box 

those sessions are shown as being "blocked".  So, is the word "block" same as "clear session"?

Also, what exactly is the difference between the two actions "clear session" and "drop" (as in drop packet)?

Thank you