Co-Located Beans

Can anybody help me answering the following question.

When can I use co-located beans meaning when can I use local references from a client.

a) Is it within the "border" of an EAR-File.

b) Is it within the EJB-Container and WEB-Containter in each case and not for example when calling a bean from a servlet.

c) Is it within the whole server-process in a Java-Cluster

(I tried but succeeded only in locally calling a bean from another bean within an EAR-File).

Thanx for anybodys clarification

Andi