Z report data is from where?

All of the Z report information comes from the Batch table...

Detailed Sales Report gets it's data from a join between several tables - the key one being TransactionEntry.

A Z report is giving the results of Exactly one Batch. There may be multiple batches per day or a batch may span multiple days. A Batch is always for a single register. A detailed Sales report is typically filtered by Date and would include sales from all registers by default. There are some values on the Z that would not be on the Detailed Sales report - like the tender break down. If you filter and group the Detailed Sales report correctly, it should theoretically match some of the information on the Z-Report, like total sales and department sales.

The only use for a Z report that I have seen is to balance your cash drawer.