Restriction / rules for bpmn symbol

this is what i try from this thread

import customRulesModules from ‘./Rules/Rule’;

this.modeler = new BpmnModeler({
container: ‘#bpmnview’,
keyboard: {
bindTo: window
},
additionalModules: [
customRulesModules
]
});

i don’t understand how to make custom rules where symbol startEvent cant connect to endEvent