There is no one single standard about the directives of the different assemblers. There are many of them.
The directives of FlatAssembler (FASM) are described in details in FASM programmer's manual, which is located on public and accessible web page.
Also, this manual should be provided for you in the downloaded package as PDF or TXT file, depending on the target OS of the package.
It covers all assembler directives and supported instructions. There are also, many examples and if something is still not clear, you can ask on FlatAssembler message board.
In the case of the questioned code, FASM code will look like this:
format MZ
entry _CODE:start
stack 200H
segment _DATA
segment _CODE
start:
mov ax, _DATA
mov ds, ax
mov ax, 4c00h
int 21h
But, considering that this program is too simple to be compiled to MZ executable (which is suitable for applications with code+data bigger than 64Kbyte) I would suggest more simple COM file format to be used:
org 100h
mov ax, 4c00h
int 21h