Stack all errors occurred and messages raised during a persistence method call. Both, Server and Session objects, has an error collection on their respective Error properties. It is normally used to get detailed information on persistence method calls results.

Query objects are used to retrieve a set of persisted instances from the database. They work much like database queries, but instead of retrieving simple table records, they retrieves complete objects that fulfill a query condition.

It is an easy to use collection class specially designed to implement one-to-many and many-to-many relationships  as of collection of simple values on your business classes. Although Visual FoxPro 8 (and up) features a basic Collection class, the fdoCollection class provides an extra number of functions and features that simplifies object relationships management, implement collection-level events, provides a way to easily access modified/added and even removed items,  optimizes memory usage and accelerate referenced instances loading with a powerful collection-level lazy loading mechanism.

When the Persistence Engine saves, retrieves or removes business objects from the data store, it can "fire" events on your business objects at specific points. Use of "Persistence Events" allows you to package all the business objects and rules in a compact, robust and efficient object model.

The FDO.FLL C++ FoxPro Link Library provides a suite of Private and Public functions. Private functions are accessed and used by the Persistence and Mapping Engines. However there are a set of Public functions you can use within your Business Classes code to access extra functionality provided by FoxDataObjects.
Those functions are extensions to the Visual FoxPro language and are mostly related to the Persistence Engine and instance persistence tracking that you can use as native Visual FoxPro functions.