In Volume Constraint quiz it stated that we can have volume constraint at top level, but my understanding is that we dont have offers at top level then why we woud need volume constraint at top level? COnceptually doesn't make sense to me.
Offers must always be created in the context of an issue and a group as they typically relate to a real world business issue and a grouping under the issue. This hierarchy helps you in defining and monitoring the metrics related the offers in their true context. For example an iPhone offer created under Sales/Handsets has a different meaning to an iPhone offer (same name) created under Retention/Proactive Retention. It is very unlikely that you would want to create an offer that will span multiple business issues and/or groups.
Volume Constraints, on the other hand, need not be specific to issues and groups. You might want to create a system-wide volume constraint that you would like to use across all issues and groups. You may recall that you can use the same volume constraint in multiple marketing programs, that is, to limit the volumes across multiple marketing programs.