So, the call SysUtils.Sleep() is not possible without including SysUtils in uses. I checked other calls, too. Mouse.GetMouseY does not compile without including mouse in uses. I definitely misunderstood what you were talking about, please expand on what you meant here:
<unit name>.<identifier> is a way to explicitly say where the compiler should look the identifier for, regardless of uses clause order. So if you have:
uses
SysUtils,Windows;
...
Sleep();
It will refer to Windows.Sleep() as Windows is placed after SysUtils. But if you have:
uses
Windows, SysUtils;
...
Sleep();
It will be SysUtils.Sleep() to call. So if you want to be sure that an identifier you call is from a specific unit, regardless of the order in the uses clause, you qualify it with the unit name.
uses
SysUtils,Windows;
...
SysUtils.Sleep();
will always call SysUtils.Sleep(), even if Windows unit also has Sleep() identifier. OTOH, you might want the totally opposite case. You want to control which identifier to call solely from the uses clause. This is a common practice in Pascal, one notable example is from fcl-web. You can have this:
uses
fphttpapp;
...
begin
Application.Initialize;
Application.Run;
end.
Just by changing the fphttpapp to fpCGI or fpFCGI, you web app magically switches to CGI or FastCGI, respectively, from embedded server.