I come from Abap development and I begin in SAP B1.
I have a question regarding user defined field.
I see that we can display those fields via user defined menu, but is it possible to add them in a "standard screen" between 2 standards fields for example ?
Yes it is possible through SDK
The SAP Business One SDK is a toolkit that enables software developers to add new functionality to SAP Business One, interface with external applications, make minor functional modifications, or simply change the look and feel of SAP Business One. It includes application programming interfaces (APIs), sample code, documentation, and development tools for SAP Business One. In keeping with the “simple yet powerful” theme of SAP Business One, the SDK is designed to be easy to learn and use, while providing a level of flexibility unmatched by any other business management application in its class.