How bpmn recognize one element?

I have one question: How does bpmn.js recognize one element?
I have found it can recognize the bpmn:startEvent because of the bpmn.json. Besides, it can also recognize the flowable:timerEvent based on the flowable descriptor which we can add in the moddleExtensions.
But if I want to define one custom Element which structure is like below:

    <bpmn:startEvent  >
        <flowable:eventType >hhh</flowable:eventType>

Can the bpmn.js recognize this structure above? How should I define it correctly?
I have defined one custom Element whose structure is like the above at custom-element-demo.
And its code is like below:

export default class CustomPalette {
  constructor(create, elementFactory, palette, translate) {
    this.create = create;
    this.elementFactory = elementFactory;
    this.translate = translate;


  getPaletteEntries(element) {
    const { create, elementFactory, translate } = this;

    function createRSE(event) {
      const modeler = document.$modeler;
      const moddle = modeler.get("moddle");
      const shape = elementFactory.create("shape", { type: "bpmn:StartEvent" });
      const flowableEventType = moddle.create("flowable:EventType", {
        "xmlns:flowable": ""
      flowableEventType.body = "";
      let extensionElements = moddle.create("bpmn:ExtensionElements", {
        values: [flowableEventType]
      extensionElements.$parent = shape;
      shape.businessObject.extensionElements = extensionElements;
      shape.businessObject.$attrs["flowable:isRSE"] = "true";
      create.start(event, shape);
    return {
      "create.registry-start-event": {
        type: "bpmn:MessageEventDefinition",
        group: "events",
        className: "bpmn-icon-start-event-message",
        title: translate("Create register start event"),
        action: {
          dragstart: createRSE,
          click: createRSE

CustomPalette.$inject = ["create", "elementFactory", "palette", "translate"];

Thank you in advance.

What exactly do you mean? Can you show what your model extension looks like?

Thank you for your reply.
My confusion is whether bpmnjs can recognize a nested structure like “startEvent>extensionElements>flowable:eventType”, because usually, bpmnjs can only handle two levels of nesting, such as “startEvent>timerEventDefinition”. Can it recognize a three-level nesting structure?

Not sure where you got this from. What do you mean by recognize? You can nest as deeply as you want.

Is it possible, to recognize it like the way you have explian?