indoor, outdoor soil question

what determines which soil is for outdoors and which is for indoors? Most brands of soil have potting soil for indoors and soil for outdoors. Is there any reason why I shouldn't use an outdoor soil indoors? The reason I ask is that i have been looking for miracle grow potting mix 21-7-14 which I can't find anywhere (even the internet) and the closest mix to that is the miracle grow outdoor soil with 15-5-10 Which I would like to try. If you are going to post about miracle grow burning plants etc. save your breath I have heard it before and I want to try it anyways. Thanks

outdoor soil isnt sterilized and can contain bugs and insects. you just dont want to use that crap and infest your op. im sure there are other reasons too but thats the main one i remember.

oh ok then i wont use it, thanks again man. I just went to the store to check the bag out it says dont use this soil in pots use MG potting soil 14 14 14 so i guess they stopped making the 21 7 14 I wanted:( but maybe i will try the 14 14 14.