Nine S Product

It's a blend of whole spices lightly roasted and powdered. Used specifically for biryani's, the masala powder adds not only flavour but the faint aroma that's required in a perfect biryani. 
The blend is similar to garam masala which means "hot spices". It is not spicy but can add pungency or over flavour a particular dish when not used in the right proportions. 
Various manufacturers sell biryani masala powder with the "secret ingredient" which makes it different from the others. 
The important thing is to get the balance of spices right- each spice which includes aromatic spices like cinnamon, cardamom, cloves, fennel seeds, staranise etc.