Your question is closely related of how the shell you are using parses user input on the command line.
If the first word on the command line is a program, located in a special folder (mostly defined by
PATH) and no more special characters are given (depends of the shell you are using), all subsequent words separated by spaces or tabs are passed to the program in a special form i.e. an array. With each word as one element in the array.
How the program, you are going to invoke interprets the arguments (located in the array) depends on how it is programmed. There are some quasi standards of how the syntax of the arguments should look like but in general the programmer is entire free. So the first argument can be interpreted as a name of a file or whatever the programmer thoughts of at the time he wrote the program.
In the case you add the special character
> to your command line, the shell dosn't append
> nor subsequent words to the array that will be passed to the program. With
> given the shell starts to make fancy things, supported by the underlying kernel (keyword piping). To grasp what's going on you must understand what
STDOUT (since it's not immediately related i omit
Everything visible you see on your terminal (in most cases a part of your display) is either written by the shell or any other program you have invoked previously to a special file (in unix everything is a file). This file has a special id and is called
STDOUT. If a program wants to read data from the keyboard it dosn't poll the keyboard directly (at least in most cases) but reads from a special file called
STDIN. Internally this file is connected to your standard input device, your keyboard in most cases.
If the shell reads
> in a parsed command line it manipulates
STDOUT in a particular kind for the time the corresponding program is running.
STDOUT dosn't point to the terminal or the standard input device any longer but rather to the subsequent filename on the command line.
In the case of the two lines
cat < file_name
the observed behavior is identical because the corresponding developer makes
cat to either read data from
STDIN or read the data from the file, whose name is given as the first command line argument (which is the first element in the array the shell passes to
cat writes the whole content of
STDIN to the terminal since we don't instruct the shell to manipulate
STDOUT. Remember that in the second line your shell manipulates
STDIN in this way, that it doesn't point to your standard input device anylonger but points to a file called
file_name in your current working directory.
In the other case of the line
man < file_name
man is not meant to read anything from
STDIN if it's called with no argument i.e. an empty array. So the line
man < file_name
man will read something from
STDIN, too if you pass
-l - to
man. With this option given on the command line you can display the content of anything
man reads from
STDIN on your terminal. So
man -l - < file_name
would work also (but be careful
man is not just a pager but also parses the input of the file and so the file content and the displayed content could differ).
STDOUT and the command line arguments are interpreted is all up to the corresponding developer.
I hope my answer could clear things up.