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( The concept can be compared to a JMS Message Queue which is provided by the runtime and can be used by all applications.)

Process Engine API

Query on deployments and process definitions known to the engine.
Suspend and activate process definitions. Suspending means no further operations can be done on them, while activation is the opposite operation.
Retrieve various resources such as files contained within the deployment or process diagrams that were automatically generated by the engine.

deals with starting new process instances of process definitions
used to retrieve and store process variables. This is data specific to the given process instance and can be used by various constructs in the process
query on process instances and executions
is used whenever a process instance is waiting for an external trigger and the process needs to be continued.

Querying tasks assigned to users or groups.
Creating new standalone tasks. These are tasks that are not related to a process instances.
Manipulating to which user a task is assigned or which users are in some way involved with the task.
Claiming and completing a task. Claiming means that someone decided to be the assignee for the task, meaning that this user will complete the task. Completing means 'doing the work of the tasks'. Typically this is filling in a form of sorts.

It allows the management (creation, update, deletion, querying, ...) of groups and users.

A start form is a form that is shown to the user before the process instance is started
A task form is the form that is displayed when a user wants to complete a form