farmOS/modules/farm/farm_quantity/farm_quantity.module

<?php
/**
 * @file
 * Farm quantity module.
 */

// Include Features code.
include_once 'farm_quantity.features.inc';

/**
 * Implements hook_permission().
 */
function farm_quantity_permission() {
  $perms = array(
    'administer farm_quantity module' => array(
      'title' => t('Administer farm quantity module'),
    ),
  );
  return $perms;
}

/**
 * Implements hook_farm_access_perms().
 */
function farm_quantity_farm_access_perms($role) {

  // Assemble a list of entity types provided by this module.
  $types = array(
    'taxonomy' => array(
      'farm_quantity_units',
    ),
  );

  // Grant different CRUD permissions based on the role.
  $perms = array();
  switch ($role) {

    // Farm Manager and Worker
    case 'Farm Manager':
    case 'Farm Worker':
      $perms = farm_access_entity_perms($types);
      break;

    // Farm Viewer
    case 'Farm Viewer':
      $perms = farm_access_entity_perms($types, array('view'));
      break;
  }

  // If the role is "Farm Manager", grant access to quantity configuration.
  if ($role == 'Farm Manager') {
    $perms[] = 'administer farm_quantity module';
  }

  return $perms;
}

/**
 * Implements hook_menu().
 */
function farm_quantity_menu() {

  // Quantity configuration form.
  $items['admin/config/farm/quantity'] = array(
    'title' => 'Quantity',
    'description' => 'Quantity configuration settings.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('farm_quantity_settings_form'),
    'access arguments' => array('administer farm_quantity module'),
  );

  return $items;
}

/**
 * Quantity settings form.
 */
function farm_quantity_settings_form($form, &$form_state) {

  // Metric or US/Imperial.
  $form['farm_quantity_unit_system'] = array(
    '#type' => 'radios',
    '#title' => t('System of measurement'),
    '#description' => t('Select the system of measurement you would like to use in farmOS.'),
    '#options' => array(
      'metric' => t('Metric'),
      'us' => t('US/Imperial'),
    ),
    '#default_value' => variable_get('farm_quantity_unit_system', 'metric'),
  );

  // Return it as a system settings form.
  return system_settings_form($form);
}