A sequence flow is a control flow between activities. The link goes from a fixed out anchor in the source object to a fixed in anchor in the target object. Some objects like Start, Choice, Split, and complex activity may have more than one output coming from the same anchor. SingleRule may have more than one output going out, each one from a specific anchor. The remaining activities may have only one sequence flow going out from one anchor except terminal activities.
You design a sequence flow using flows in the BPM. The constraints existing on outgoing flows are verified when you check your model.