someone has experience using 800XA like OPC server for third party clients,
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If you talking about AC 800M OPC or MOD300 OPC server - yes, both can be used by the other DCS/SCADA OPC clients compatible with OPC DA protocol. All standard data types (bool, integer, float, string) are available if no special access restrictions were specified in the user program.
Otherwise I don't understand the question: 800xA is the automation system, OPC server are just one of the services providing interface between controller and automation system... please clarify the question then.
800xA supports 3rd party clients such as cpmPlus Smart Client but that doesn't call "OPC".
Third-party controller can be connected to the 800xA System using PLC Connect, but 800xA in this case acts as OPC client for the OPC server (supplied with PLC, for example Matrikon OPC Server).
Historical data and logs collecting by the 800xA system stored in databases which are accessible from ODBC or another dedicated DB engine interface. In this case 800xA acts as data provider/collector. Not the OPC server.