I was trying the BPMN online editor for the “What if you are hungry?” process. The exported BPMN diagram does not have XML namespace prefix for definitions tag. Later, I was working on a different process. When I exported that one, I could see that the tag as <bpmn:definitions ... IN case of former, I see there is a default xmlns definition to the same URI as that of bpmn prefix. So, I think, technically, documents are same. Nevertheless, I wanted to know if there is a predictability around when a tag is prefixed or not.
1st Case
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI" xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="sid-38422fae-e03e-43a3-bef4-bd33b32041b2" targetNamespace="http://bpmn.io/bpmn" exporter="bpmn-js (https://demo.bpmn.io)" exporterVersion="8.8.0">
2nd case
<bpmn:definitions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:bpmn="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:dc="http://www.omg.org/spec/DD/20100524/DC" xmlns:di="http://www.omg.org/spec/DD/20100524/DI" id="Definitions_16be12s" targetNamespace="http://bpmn.io/schema/bpmn" exporter="bpmn-js (https://demo.bpmn.io)" exporterVersion="8.8.0">