The main idea of srv4pos is to exchange data between POS terminals in a various ways:

Data domain is very simple:

«Commodity» is an object-link between Category and Product.

Companies are isolated from each other:

Every company is designated by so-called sellerId and has Unique Resource Location:

Desired API version can be choosen from Release notes.

Where to start? Choose business case for your needs at API chooser

Technical guides per laguage are at: Java, .NET, JSON-REST.

And make sure you aware about Error handling.