First things first:
A- represents the number of contacts in that segment
Why is it grey in colour? Because contacts have flown out of that step and into the next one
B- represents the number of contacts who did not get sent the email
Why is it green in colour? Because contacts were not sent the email and are still in that step
Why were they not sent the email? Double click to find out.
When you export this report, the status is displayed in terms of exclusion codes, like so:
Following is an intepretation of all the Status Codes:
Now, you know.