Hello,
it depends on which functionality you need. Simply said - to build .bpmn2 definition and start process from it, you'll need jbpm-bpmn2 artifact :
<dependency>
<groupId>org.jbpm</groupId>
<artifactId>jbpm-bpmn2</artifactId>
<version>5.4.0.Final</version>
</dependency>
Usually you need other artifacts:
jbpm-persistence-jpa ... to create knowledge sessions which persist their state to database
jbpm-bam ... to log engine activity
jbpm-human-task-hornetq|mina|core ... to be able to interact with human task service via HornetQ/Mina protocols or by using LocalTaskService