Carry forward of business losses allows set off against future business profits subject to a statutory time limit. Losses under Profits and gains of business or profession (excluding speculation business) that cannot be set off under other heads are to be carried forward and set off against future business profits; any unabsorbed portion continues to be carried forward. A statutory time limit restricts how long such losses may be carried forward. Where a carry forward of an allowance is also involved, this provision requires that business loss carry forward and set off be given effect to first.