What onDynamic really do? reading the docs, still don't understand when onDynamic called

import { revalidateLogic, useForm } from '@tanstack/react-form'

function App() {
  const form = useForm({
    defaultValues: {
      firstName: '',
      lastName: '',
    },
    validationLogic: revalidateLogic(),
    validators: {
      onChange: ({ value }) => {
        if (!value.firstName) {
          return { firstName: 'A first name is required' }
        }
        return undefined
      },
      onDynamic: ({ value }) => {
        if (!value.lastName) {
          return { lastName: 'A last name is required' }
        }
        return undefined
      },
    },
  })

  return (
    <div>
      <p>{form.state.errorMap.onChange?.firstName}</p>
      <p>{form.state.errorMap.onDynamic?.lastName}</p>
    </div>
  )
}

import { revalidateLogic, useForm } from '@tanstack/react-form'

function App() {
  const form = useForm({
    defaultValues: {
      firstName: '',
      lastName: '',
    },
    validationLogic: revalidateLogic(),
    validators: {
      onChange: ({ value }) => {
        if (!value.firstName) {
          return { firstName: 'A first name is required' }
        }
        return undefined
      },
      onDynamic: ({ value }) => {
        if (!value.lastName) {
          return { lastName: 'A last name is required' }
        }
        return undefined
      },
    },
  })

  return (
    <div>
      <p>{form.state.errorMap.onChange?.firstName}</p>
      <p>{form.state.errorMap.onDynamic?.lastName}</p>
    </div>
  )
}