China DOS Union

the drive specified by drive letter D:.
I don't quite understand the part “the drive specified by drive letter D:”, please help me out.
2. The command MSDOS.SYS>PROMPT OK&g
gives this result: OK>
What is the purpose of this? Why change the current directory to “OK>”? I really don't understand, please help me out.

From using sys /?, there is no such thing as what you said, “the drive specified by drive letter d:”.
Instead it is as follows:
Copies MS-DOS system files and command interpreter to a disk you specify.

SYS drive2:

Specifies the location of the system files.
drive2: Specifies the drive the files are to be copied to.
For example: you just need sys a:\ c: and you can make the disk into a DOS boot disk.

prompt　　　sets the prompt

［When to use it］　　When you are tired of the c:\> prompt, or if you want your prompt to be different from others,
　　　　　　　　give it a try. It's a very interesting DOS command, and it can display the time and date at any time.

［Usage］　　prompt $p$g uses the current directory name and the > sign as the prompt; this is the most commonly used prompt

　　　　　　　　prompt $t means time　　　　　　prompt $d means date

　　　　　　　　prompt $$ means $ 　　　　　　　prompt $q means =

　　　　　　　　prompt $v means the current version　　　　prompt $l means prompt wang$g uses wang> as the prompt

　　　　　　　　WANG>prompt $t$d$g 　uses time, date, and the > sign as the prompt

　　　　　　　　0:01:07.77Thu 08-29-1996>prompt $p$g

　　　　　　　　C:\DOS>

Note: excerpted from a Hongen tutorial

The command C：\>PROMPT OK&g you mentioned should be C：\>PROMPT OK$g, right?
Its function is just to always display the path name you are in as ok>
Personally I don't think it has any benefit. (Just my personal opinion.)

Change it if you want, it's just personal preference

Many thanks for everyone's guidance, I understand a lot more now.
When you have time later, let's chat more. My QQ is 58280444, from Wuhan.
It's hard being away from home, thanks everyone for looking out for me.

I'm from Wuhan too, but I'm not there now.