Starting from 7.5 we have changed the way Images are captured and deployed.
Now there is a separate site server component for PS, that will make it also a DSPS so it will have deployment solution packages on it.
So the basic logic will use the same as normal packages.
Client will request for package from NS.
NS will provide the codebases it has for the client base on subnet and what DSPS it has for this subnet client is requesting from.
If there is a DSPS then it will be provided.
If there is no available DSPS that has the package NS will return its own codebase for the client
When 7.5 was released Support created the following document to better understand how it works.
https://support.symantec.com/en_US/article.TECH215008.html
NB: this article is for 7.5 the logic has been improved a bit in next releases