Create SAP Source system

I have some questions because I didn't succeed to.

1, The SAP source system should not limit to any client or server of system.

2, I choose a RFC destination, and what does user of "Background user in source system" and "Connection of Source system to BW" ?

I have read the help online (library) but didn't get any details information. And it seems that the IMG menu is not same as help said.

3, Why repeat the password?

Then got message " please log on as administrator in the following screen".

Administrator means SAP*? I have SAP_ALL, SAP_BW_ALL profiles,

User already exists in Source system, Only continue if profile and password is correct.-->Continue

RFC destination already exists in source system, do you use the destination, check or cancel--->use

goes to <b>RFC destination which is not the one I use</b>

Mssage:"RFC user Login failed. Check the connection"

at the end, "RFC destination BW could not create in the source system"

I'm puzzled by the error message, anybody can explain to me ?

helpfull answer will be rewarded.