The number “2” indicates what and where is the output going?

Question

1. Every time I run the command “dostuff” I want to capture the output into file and any errors into another file. I use the command “dostuff > A.out 2> B.out”. in this case, the number “2” indicates what and where is the output going?

Answer 1

2 here refers to stderr, so this is redirecting everything that would have gone to stderr to the file B.out.