Altiris Agents are never permitted to download packages across a hierarchy, they will always wait and download from local Package Servers
Only Package Servers in the same Site as the NS are permitted to download packages across the hierarchy
The user controls which Package Servers in the NS site they do not want to download across the hierarchy by setting these Package Servers to 'Constrained'. As Constrained Package Servers are not permitted to download packages outside their hierarchy location
In relation to the above point, there is also an NS core setting called 'PackageRequestBlockoutSeconds'. Below is an explanation of how this setting works.
Other KBs to check:
HOWTO9693 - When replicating a package, what exactly is replicated?
HOWTO9694 - How does a replicated package get distributed to child Package Servers?