I believe inward means directed towards the inside of the building, so loading based on Se (+). That would make sense for form deck trying to determine allowable spans for concrete. I think I checked that once in the past and figured out that is what it's based off, but I'd have to double check the numbers again to be sure.
I agree with straub46 and would define it as shown below. Some of the reasons that this orientation is defined this way include:
1) For floors, this is the most efficient use of the concrete volume.
2) For roofs this maximizes the attachment surface available for the roofing.
3) For low uplift roofs and floor deck under wet concrete, this prioritizes sagging moment flexural capacity at end span conditions which often govern the deck design.
So:
a) For a typical roof application, inward points towards the interior of the building and;
b) For a typical floor application, inward points towards the interior space below the floor.
