OPC License & OPC HDA(History Data Access)

Hi,

We are planning to connect DCS melody S+ Operations to a third party software. I just wanted to confirm how does OPC licensing work. We have one user OPC license so do we need to purchase license for each third party connection? or if just the OPC is enabled then we can connect as many systems as we can?

Lastly, if our operational and history server is combined and if we try to implement OPC HDA(History Data Access) package to extract complete history data then will the complete existing history data be available in the OPC HDA or Only the data that is being generated after establishing OPC HDA will be stored in HDA server?

Thank you in advance for your response.