Great question! They are not actually the same thing. The cream cheese sold in the tub has a much softer consistency, suitable for easily spreading on a bagel or sandwich. The much thicker bar-style cream cheese is what is typically called for in cream cheese frosting. You can use the cream cheese in the tub, but you will have to alter the quantities of the other ingredients. When I lived in London, where I could only find the tub cream cheese, I would reduce the amount of liquid (usually milk) in the frosting recipe, or eliminate it entirely. I also would sometimes reduce the amount of butter in the recipe as well, especially if I was planning on leaving the frosting or frosted cake out at room temperature for a while, since at room temperature, butter is very soft and would cause the frosting made with tub cream cheese to slide off the cake or wilt on cupcakes. I preferred to reduce the butter and liquid, rather than increase the amount of confectioner's sugar (which would have the same stiffening effect on the frosting), because I do not like it when cream cheese frosting is very sweet. If you can only find tub cream cheese, I suggest you experiment to figure out what ratio of ingredients you prefer, based on how thick and sweet you like your frosting. Best of luck!