so if i not select the checkbox i will have the same id on the same machine or i will have the same id on every machine?
An Internet-ID code cannot be the same on every machine. Each machine always has its own unique ID.
then if i select generate new id it changes.. but not on the same machine but on another machine? is this right?
It changes relative to the old Internet-ID which was used previously on the same machine. Let's say you ran Agent on computer A and ID was 111-111-111-111. Then you configured a new Agent with the enabled option "Generate a new Internet-ID code" and ran this Agent on computer A. In this case the old ID will get re-written to a newly generated ID (let's say 222-222-222-222).
the important is that i not have the same id on multiple machines
You won't have same IDs on multiple machines anyway. You can be sure of that.