Negative Posting

Hi folks,

I appreciate once again for all your dedicated help and efforts.I have 2 questions and i need you guys to help me on this.

I have come across 'Negative Posting'. can anyone of you please let me know what is 'Negative Posting' and what affect would it shows in what level of functionality?

what are the general erros comes up in Idocs which are related to SD? 850/875 , 810 , 857 or 858.

Thanks in advance ,

solutions will be rewarded,

Kanna.