Function 03CH creates the file and it is open on exit of the int 21.
The "5" that you see is the file handle for accessing it. Use the regular write/read operations to use it.
This function works under real-mode DOS and a DOS window.
If you are running under a DOS window, you can also use function 071H and 06CH for extended file functions and long file names.
If you are using these functions - it is best to try a File Open before using File Create. If the File Open fails, meaning the file does not exist- then use File Create. File Create will always create a new file, if a file by the same name already exists, it is gone.
I looked at some of your prior posts- and see that you used an Arduino with a Vortex 86EX
I've used the VortexDX since 2011, and am currently running a VortexDX3. I've just started using the Watcom compilers and assemblers, which can generate code for DOS and DOS extenders.