DDE string - {id}? Web ID to represent symbol code?

Hello all,
For constructing DDE string, we can put a unique id {id} to represent the ticker. I initially thought it is the Web ID. But the documentation says:

{Id} - evaluates to unique ID (running counter incremented by 1 with each symbol)

I could not understand what it really means. Appreciate if someone could explain it.

Also, apart from {Ticker}, is there a way to use Symbol Alias or Web ID to represent the stock code?

thanks.