If you're very careful, you could try substituting passata for tinned tomatoes in a recipe.  For example, if a recipe calls for 300mls of tomatoes and you have 300mls of passata, then theoretically, they should be interchangeable.  However, passata is a cooked product, so reduce the amount of liquid added to your recipe. For example, if a sauce calls for 500mls of water, you should only use 300mls of passata plus another 300mls of water.  If your recipe calls for fresh tomatoes, it is not advisable to substitute them with passata.