A resource manager connection factory reference may be scoped to a component but multiple components can reference the same connection factory - i.e. you define a single datasource in your app server then reference it from multiple components.
When a connection is marked as sharable it can be reused by multiple components executing in the same transaction. For example EJB1 starts a transaction, obtains a
JDBC connection, uses it, and then invokes a method on EJB2. EJB2 also uses a JBDC connection with same connection properties as the one used EJB1. If the connection is shareable EJB2 will reuse the same connection, as opposed to creating a new one.
Check out the JCA spec if this still isn't clear.
SCJP, SCWCD, SCBCD, SCEA 5