What is the meaning of various sements of gl code combination eg. reporting entities, cost center , natural accout etc.. do they have any relation with any organization entities or they r just numbers to identify accounts..
eg is reportint entiity 5500 will be any organization unit?

Yes all the segments have proper relevance.

For your accounting combination, you can set one account as natural account, one as balancing segment account. That will be used to do your accoutning and to balance your entries with tose accounts.