Field help (PIHCCE1) 

When entering an invoice line, the analytical dimension are initialized based on the setup of default dimensions (PID code). These dimension codes can be modified manually if authorized by the transaction.

If the invoice line originates from a document line, the analytical dimension types are loaded by the analytical dimensions on the source document.

If analytical dimension types are not specified on the source document, an attempt to initialize these analytical dimension types is made according to the Default dimension code.
When an analytical dimension is used for project management tracking, any modification of the project code on the document line leads to the dimension code being reset based on the Default dimensions setup.

In the case of an inter-site or inter-company invoicing, analytical dimension types can also inherit from the sales invoice dimension types, based on the default dimensioncode on the invoice line (PID).