i think that a simple aapproach to it can be taken
1. for current year loss:
The loss from non speculative business can be set-off against any income except salary.
2. for b/f loss:
The b/f loss from non-speculative business from previous years can be set off against non-speculative and speculative income only.
so to conclude we can say...........
a. that b/f loss can only be set-off against income from same head (including all sources of the head).
b. that the current year loss can be set-off from income of same head as well as other heads except salary.(including all sources of heads).
further all of the above explaination applies only for loss of NON-SPECULATIVE business only.