Difference between demi-glace and beef stock. Can I use a demi-glace instead of a beef stock? Making of demi-glace is well defined for me as a product of veal bones and vegetables reduced to gelly concentrate in temperature 5-10C. 

Here is my demi-glace which I refer to:



Beef stock, on the other hand, has ingredient similar to demi-glace but some recipes refer to only veal bones and some to bones and meat. The final product is less reduced than demi-glace like this:



So my question is:

If some recipes refer to use beef stock as an ingredient (for example onion soup) can I use my demiglace instead? If yes, what proportion I should dissolve my demi-glace with water to achieve beef stock substitute? Also if this recipe involves reduction after adding beef stock is a point of dissolving my demi-glace with water?
demi glace is beef stock seriously reduced. So yes you can use it just need to add a lil extra water. What proportion no idea depends on how strong a flavor your wanting really. Just put some demi glace in a sauce pan (low heat) let it melt, add water till you get the right consistency/flavor your looking for.