All of the QoS collected by probes goes into the RN_QOS_DATA_* tables. Each table represents a "type" of QoS, such as QOS_CPU_USAGE or QOS_MEMORY_USAGE. To see a list of QoS types known to your NMS, you can look in the S_QOS_DEFINITION table. The QoS definitions are created by the probes, so as you use new types of probes, you will see more definitions appear in that table.
Within each of the RN_QOS_DATA_* tables, you can have data from multiple sources and/or targets. For example, the table that holds the data for the QOS_CPU_USAGE type may have CPU utilization data from many hosts/devices. A QoS object is uniquely identified by a type, a source, and a target. You can see a list of all QoS objects known to your NMS by looking in the S_QOS_DATA table. For each QoS object, that table stores the ID and which of the RN_QOS_DATA_* tables contains its data.
I hope this helps as a starting point.
Here's a doc that might help you with the Db schema.