How does the demand propogate from DC to Plant

Hello Guru's,

                          I have a very basic question . I have a DC and a demand on the DC , the DC is connected to the plant using Tlane . Now when i run SNP irrespective on heuristics, optimizer or CTM .

1) How does the engine work and how is the demand propogated to the plant .

2) What are the ATP categories involved in the process .

Regards,

Nick