Complex Gateways in BPMN are typically used for splitting/deviating or merging/ converging a complicated process flow scenario. Users use this only when the more well understood parallel, inclusive, and exclusive gateways cannot handle a situation.
The anatomy of a complex gateway can be represented by a combination of Exclusive, Parallel, Inclusive gateways. The complex gateway is an advanced concept used by users with more experience in using BPMN notations. Using complex gateway makes the process map look more compact/lean as well.
- Exclusive Gateway (XOR) - where only one path is followed (a binary situation)
- Inclusive Gateway (OR) - where one or more paths can be followed
- Parallel Gateway (AND) - where both paths are followed in parallel
- Complex Gateway - where multiple inputs are processed and based on certain condition/rule a merged/converged output is created for the next activity.
Hope this helps.