Chapter 43. Redirecting Input and Output of Unix Power Tools, Third Edition has to say this about cat command:
Standard input (stdin) normally comes from your keyboard. Many programs ignore stdin; you name files directly on their command line — for instance, the command cat file1 file2 never reads its standard input; it reads the files directly. But without filenames on the command line, Unix commands that need input will usually read stdin. Standard input normally comes from your keyboard, but the shell can redirect stdin from a file.
Ok but happens when we just type
cat > filename in the command line? Isn't cat reading from the stdin and stores that that into file "filename"? Is the above excerpt from the book just saying that only the particular form of using cat with a FILE argument never reads from the stdin?