The standard Exact Online API documentation is available at developers.exactonline.com for the REST APIs. When the page does not display the documentation, please try again using “Incognito Mode” of your browser.
For XML APIs, it is best to download the XSD from the top-left menu option “Import/Export”.
The additional properties such as data type and nullable can be found in the Invantive Exact Online data model documentation.
The relationship between the Exact Online API tables can be found in the graphical data model:
More details on establishing an Exact Online API can be found in How do I create an Exact Online API interface?.