There are four ways of connecting flow objects to each other or to other information within a process. What connection type has to be used, depends on the objects you want to connect.
Sequence flow connects flow objects like events, activities and gateways. It defines the order of performance of the flow objects. Each sequence flow connection has only one source and only one target. During performance of the process, a token leaves the source flow object, traverse down the sequence flow, and enter the target flow object.
Depending on its source object, a sequence flow can have a conditional expression attribute. The conditions are associated with an exclusive choice gateway. The condition expression must be evaluated before a token can be generated and then leave the source object to traverse the flow.
More information: Connecting Flow Objects in a Process .
Data flow connections define where data objects are used in your business process model. You need the data from data objects to define the data changes within an event or an activity. Data mappings show how data used as input and output by the activities and the events in your process is transformed. The data objects represent the process context.
An association connects additional information like a note to individual elements in the process model, such as flow objects, swim lanes and data objects. Such notes are represented by annotations.
More information: Using Annotations in the Process Model
A message flow is used to show the flow of messages between two participants that are prepared to send and receive them. According to BPMN, in SAP NetWeaver BPM , two separate pools in the diagram represent two participants in the sense of different business entities.
More information: Connecting Separate Process Participants with Message Flow , Pools and Lanes