that is expected behavior as signals are broadcasts meaning all process instances listening to given signal will be triggered. There is a jira on road map to include scopes to allow to signal individual instances but currently this is not available. You could workaround this by using either script task or send task (with custom work item handler) that will signal using ksession.signalEvent(signal, event, processInstanceId)
Thanks for your answer, Maciej!
Can you tell me something about throwing and catching error events to accomplish my goal?
I was not able to add a catch error event to my process, because a "node has no incoming connection was issued in the designer, but I was not able to draw such a connection (the flow object disappears ad sono ad I release the mouse), even using a gateway.
Can you explain me how to use these objects?