I think Verification can occur anytime during a project. I do not have any industry experience but theoretically speaking Verification can occur at any point. It can also occur during the research phase where making a change in your project is a lot easier versus the execution phase where it would require a lot more changes in your timeline and paperwork. So to sum it all, it is not a rigid rule it can occur into other processes. And the advantage of this is, that one can use this verification process according to their project. Not every project is the same and not every project requires the same set of procedures.

Just to add to everyone else, verification is not always as described theoretically. From speaking with individuals working in the field, every company has a different set of rules or loops to full-fill certain GMP or GCP.