The basic workflow for creating a part is the following:
Create a new document, create a new 'Model' with the macro 'new_Model'. This will create a new App::Part called 'Model' with the default structure
Create or import geometries in bodies (PartDesign::Body)
Create LCS with the macro 'new_ModelLCS', and place them wherever you feel they're useful. Use the MapMode to attach the LCS
Save part (only saved parts can be used currently). If necessary, close and re-open the file
Insert an External Part : creates a FreeCAD App::Link to an App::Part in another document. Only parts from documents already saved to disk can be used. If there are multiple parts in a document, they can be selected individually. A part can be inserted (linked) many times, but each instance must have a unique name in the assembly tree. If a name already attribuated is given again, FreeCAD will automatically give it a unique (and probably un-user-friendly) name.