Or

At least one condition must be met.

You can use or object to connect two conditions. If either of of fields with name a and b will have value x condition is met.

{
  or: [condition1, condition2, ...] // (condition1 OR condition2 OR ...)
}

{
  fields: [{
    name: 'Or condition',
    component: 'text-field',
    condition: {
      or: [
        {
          when: ['a'],
          is: 'x'
        }, {
          when: ['b'],
          is: 'x'
        }
      ]
    }
  }]
}

As the value you have to use an array of conditions.

Also, you can use a shorthand:

{
  fields: [{
    name: 'Or condition',
    component: 'text-field',
    condition: {
      when: ['a', 'b'],
      is: 'x'
    }
  }]
}