An activity represents work that a company or organization performs using business processes. An activity can be atomic (it is not broken down into a finer level of detail) or non-atomic. Atomic activities are represented in the Process Editor by tasks. For more information about how BPMN defines activities and tasks, see
http://www.bpmn.org.
User tasks are those that require human interaction with a software application. For example:
Manual tasks are those that are completed by a person without using software. For example:
Service tasks can ideally complete without human interaction (for example, an automatic email notification or a web service). For example:
Script tasks contain a set of instructions written in a scripting language (usually added to the step by the solution engineer) that will be executed in the runtime environment when the process is deployed and executed. For example:
Send tasks are used to send messages to a system or person outside of the process (often using a web service):
Receive tasks are used to wait for a message from a system or person outside of the process (often using a web service). This type of task can be used to start a process as long as it has no incoming sequence flow and there are no start events in the process:
On the Interface tab, you can add parameters to a receive task; however, you cannot add data fields because data fields are used internally in a process, and parameters are in this case, inputs from an external process (for more information, see
Creating Data Fields and Parameters).
Reference tasks refer to another task and prevent you from having to duplicate the same task several times in a process.
You can create a reference to another task using either the Reference Task gadget on the reference task, or in the Properties view for the reference task.
To create a reference to another task on the General tab of the Properties view for the task, complete the
Referenced Task field by clicking

and selecting a task from the list:
After you have a created a task reference, clicking Go To selects the target of the reference task and brings it into the visible part of the Process Editor (it also displays the Properties view for the task). To return to the previously-selected task, click

on the toolbar.