The using statement defines a scope at the end of which an object will be disposed.

They die when they are pulled off the stack (go out of scope).

A value specified in an enterprise bean's deployment descriptor that is used by the EJB container to control the transaction scope when the enterprise bean's methods are invoked. A transaction attribute can have the following values: Required, RequiresNew, Supports, NotSupported, Mandatory, or Never.

A scope over which the same security mechanism is used to enforce a security policy. Multiple security policy domains can exist within a single technology domain.

A scope over which security policies are defined and enforced by a security administrator. A security policy domain has a collection of users (or principals), uses a well-defined authentication protocol or protocols for authenticating users (or principals), and may have groups to simplify setting of security policies.

Stack and heap are memory sections of the .net application. The CLR(Common Language Runtime) is in charge of managing those sections. Value type variables are stored in the stack where storing order is Last in first out. When a variable is out of scope it basically fall off the stack. Reference type variable are stored on the heap where storing order is First in first out. When a variable becomes out of scope it is mark for collection. The GC(Garbage Collector) is the part of the CLR responsible for cleaning up the heap releasing resources occupied by Reference type variables.