What the difference between Checkout and All Pages?

[wssupport]

If you choose Checkout, the clients must verify their email before it is stored in WHMCS DB. That is good if you have WHMCS Plus or Starter package.
 
If I choose All Pages, that means they purchase first and then verify their email once the order has been done.

If installed will my clients need to verify again?
You have to choose -- click on 1) Bulk Verify Existing clients and it will part all existing clients or 2) Let your clients verify themselves.