How can each customer ID have multiple unique visitors? I guess each customer when they sign in using specific username and password they are identified with customer id and it has to be 1 unique visitors right ?

I am trying to get report for the below mentioned criteria. Please tell me should i create segment or calculated metric and on what logic ? Registrants 1 [reg 0-10 days ago] Registrants 2 [reg 11+ days ago - 12mos]   Registrants 3 [13+ mos ago since you registered] Buyer 1 [1 purchase & you've purch...

What is the meaning if I assign an event = 1 in a segment? like show in images 

Is there any resource where we can learn about creating segment? especially advanced segmentations?