write

Formatting depends on localization. About localization read Localization
The main formatting field is date.
For that it is used properties: edit, read, write.
Another extra property is inputFn.
inputFn formats displaying value.
inputFn is used for almost all fields.

Example: Date formatting

...
items: [{
  type: 'date',
  label: 'Birthday',
  name: 'birthday',
  emptyText: 'm.d.Y',
  format: {
    read: 'm.d.Y',
    write: 'm.d.Y',
    edit: 'm.d.Y'
  }
...

Example: inputFn

...
items: [{
  type: 'number',
  label: 'Salary',
  name: 'salary',
  emptyText: '$80,000',
  format: {
    inputFn: function salaryInputFn(value){
      value = value.toString().replace('$', '').replace(/\,/g,'').replace('-', '').replace('.', '');
        
      if(value.length === 0){
        value = '';
      }     
      else if(value.length > 6){
        value = value.substr(0, 6);
        value = '$' + value.replace(/\B(?=(\d{3})+(?!\d))/g, ',');
      }
      else{
        value = '$' + value.replace(/\B(?=(\d{3})+(?!\d))/g, ',');
      }
      
      return value;
    }
  }
...

Default

'm/d/Y'