what is the business criteria for APO demand planning and r3 demand planning

hi men,

it would be nice to have your inputs /clarity on Demand planning .i hav worked with both APO demand planning and R/3 demnd planning md61 and sop stuff...frm the configurator angle..but in terms of business under wht business circumstances or scenario we use APO DP and r3 DP .....because my clint has both R/3 and APO and i want to decide how the demand has to be addressed ..it has to be addressed by APO dp or r3 demand...would be nice to hav clarity

thanks