doubts in sd

hi gurus,i have some doubts in sd,can u people pls brief me on that.

1.How will access the data in to production

2.If for example, unique document number ranges are required for plant and billing docu.type. How to create this user exit?

3.what do you mean by cross implementation.

thanking u in advance.points will be rewarder.

regards,

ravisankar.p