How to understand/use GDT, MDT, namespace?

[cain_sun]

hi experts,

I am going to develop an inbound sync service for Cloud to create/change existing accounting document in ERP. When I am reading the concepts in PI, about enterprise service builder, these questions come up in my mind.

Data Types:

1. in the link Data Type Projection, it mentioned that we should define a maximal GDT and a maximal MDT for my business object, and then create separate MDT based on it. Here comes the questions:

1.1 for my scenario if I am going to generate services like Create accounting document and Change Accounting Document, I should create one maximal GDT/MDT for accounting document, and separate MDTs for each of them?

1.2 The folder Global Data Typesin the namespace http://sap.com/xi/SAPGlobal/GDT of SWCV SAPGLOBAL 2.0 of software component SAPGLOBAL contains all the SAP-wide valid GDTs? These GDTs mean the semantic types valid in all SAP-wide? so the Global Data Types folder in my local NS is a sub set of it? What is the difference between the folder Data Type and Global Data Type in a namespace?

1.3 How can I know whether to create a global data type or a local data type during my development? If a data type is potentially possible to be used in other scenario, I should create in the global data type? In my case, may be the accounting document header or item structure can be a GDT?

2 I saw the development for service is in X3E system. Is X3E a dev system for PI? There must be test and product system for it. What is the transport mechanism for them? Or they will be image-copy among them automatically?

Thanks for reading