The  system configuration is important tool for creating global value used to customize or setup tasks 
, in OroCRM you can find the technique of how creating in official documentation :
http://www.orocrm.com/documentation/index/current/book/configuration 
here ibnab giving you other way to create and get value from  system configuration .
By ui_only: true and Configuration Provider  , Yes but what is  this ?
`ui_only` - indicates whether a field is used only on UI and do not related to any variable (optional, defaults to false)  = if you have a field doesn't need variable values or default value you can use   ui_only  
other  property of field :
// 'data_type' be specified for all fields except 'ui_only' ones
Field declaration have required property `type`.
`type` - refers to form type of which field should be created
`tooltip` - show additional info about field
`acl_resource` - determines acl resource to check permissions to change config field value(optional)
`priority` - sort order for displaying(optional)

Create inside YourVendor/YourBundle/Resource/config/system_configuration.yml

 
oro_system_configuration:
    groups:
        yourvendor_yourbundle_manager_setting:
            title: yourvendor_yourbundle.system_configuration.groups.settings.title
            icon:  'icon-file'
       yourvendor_yourbundle_manager_view:
            title: yourvendor_yourbundle.system_configuration.groups.view.title</p>
 
<p>    fields:
        yourvendor_yourbundle.allow:
            ui_only: true
            type: choice
            priority: 70
            options:
                label: yourvendor_yourbundle system_configuration.fields.allow.label
                required: false
                multiple: true
                choices:
                    marketing: yourvendor_yourbundle system_configuration.fields.marketing
                    activities: yourvendor_yourbundle.system_configuration.fields.activities
    tree:
        system_configuration:
            platform:
                children:
                    general_setup:
                        children:
                           yourvendor_yourbundle_setting:
                                priority: 400
                                children:
                                   yourvendor_yourbundle_view:
                                        priority: 100
                                        children:
                                            - ibnab_pmanger.allow
 

All explain is inside the official documentation in top , but the more important part  for our tutorial is :

 
    fields:
        yourvendor_yourbundle.allow:
            ui_only: true
            type: choice
            priority: 70
            options:
                label: yourvendor_yourbundle.system_configuration.fields.allow.label
                required: false
                multiple: true
                choices:
                    marketing: yourvendor_yourbundle.system_configuration.fields.marketing
                    activities: yourvendor_yourbundle.system_configuration.fields.activities
 

See the ui_only: true, and options with  property  choices contain two static value and is multiple choice , we finished  , you don't need to redeclare  inside DependencyInjection/Configuration.php because oro in  vendor/oro/platform/src/Oro/Bundle/ConfigBundle/DependencyInjection/SystemConfiguration/ProcessorDecorator.php 
test on two level :

 
                if (!isset($this->variables[$varName]) && empty($varData['ui_only'])) {
                    throw new InvalidConfigurationException(
                        sprintf(
                            'The system configuration variable "%s" is not defined.'
                            . ' Please make sure that it is either added to bundle configuration settings'
                            . ' or marked as "ui_only" in config.',
                            $varName
                        )
                    );
 

if is variable and declared inside Configuration.php continue ,if not see if have  ui_only if not  throw  InvalidConfigurationException .

Ok now what about   Configuration Provider for example I want use my the value of this field outside controller , here I need create  Configuration Provider and inject to my target class .

1 – Create my provider inside our services.yml I can pass as argument to any service @oro_config.global:

 
        arguments:
            - @oro_config.global
 

is object of type Oro\Bundle\ConfigBundle\Config\ConfigManager give you the ability to use the function get('pathtoyourfield');
for example the construct of our class is :

 
 protected $configManager;
    public function __construct(ConfigManager $configManager)
    {
        $this->configManager = $configManager;
    }
 

   Now you can use $this->configManager->get('yourvendor_yourbundle.allow') for getting vlaue