Some recipes call for powdered sugar/confectioners sugar, which I have found in 2 lb bags. Others call for 10x confectioners sugar, which I have only found in the small box. Can one be substituted for another, or is there a major difference between the 2?
In my experience powdered sugar is powdered sugar. I don't sift either.
They're the same thing.
There is also a 6X powdered sugar, mostly used by the manufacturing trade, although I've seen it for sale at GFS.
The X refers to the fineness of the sieve that the sugar goes through to become powdered.