Internals covers deep internal operation and is not introductory in nature. If you are interfacing to Fieldpine you are probably more interested in Technical / API information. Most deep internal documentation is restricted distribution and not available online.
Details about structure and use of physical SQL database tables
Details of how POS operates in a virtualised server
Sample datasets for research purposes
Projects that developers are working on
Full Database search architecture
Formal definitions of Business Transactions (TUBT)
How online sales are configured and processed