Q:

What does it mean by $scope=global in ATG..?

According to the documentation,

Global: Component is shared among all users. 
  Session: Separate instances of the component are provided to each user.

Is that means, for global component, there is only one instance for the whole nucleus system..
If this is true, how does it valid for components like ‘/atg/dynamo/transaction/TransactionManager’ and most of the droplets..?
Because those components are used by several users at a single moment
Edited:
I understood the ‘TransactionManager’ behavior. According to the definition there should be single transaction manager, and he should keep transaction objects per each transaction.
But my question is still valid for droplets like, foreach, switch, etc (most of them are globally scoped)
If there is only one instance of corresponding class for the whole nucleus system, isn't it having bad effects on performance?

A:

Historically there were three different types of scope available in ATG. This has now increased to 5, with the addition of Window (generally only used in the CSC application so try not to use it) and Prototype (added to support the use of Endeca Cartridge Handlers).
As you highlight from the documentation, global component is instantiated once and is shared by all users, while a session component is created once for a given session and is shared by all requests of that session. Similarly a request scoped component is freshly instantiated for each request that uses it. 
From a performance point of view, resolving the path to an existing component (for example a globally scoped component like ForEach) takes a little time, but instantiating a new object (in other words a request scoped component) is comparatively more expensive. 
So in the case of a ForEach droplet it gets instantiated once but in the service method it actually extracts the parameters from the request:
String elementName = pRequest.getParameter(ELEMENT_NAME);

This means that your globally scoped component is thread safe in that it only takes in parameter from the current request. So in general, if a component can be shared by multiple users, without worrying about synchronisation, it should  be globally scoped versus session or request scoped. (The rule of thumb should be that if your droplet is request scoped, you are likely doing it wrong). 

