OroCommerce give ability to start checkout from multi source like shopping list or order itself by re-order , So the entity source for checkout should implement CheckoutSourceEntityInterface , like Order and ShoppingList entities . And Checkout entity has relation with entity CheckoutSource. 

You can start create you custom source of checkout by adding relation between you custom entity and  CheckoutSource entity by migration , let’s give you custom entity a name of CustomSource
Example of your full class migration:

 
<?php
namespace Ibnab\Bundle\CustomBundle\Migrations\Schema\v1_1;
use Doctrine\DBAL\Schema\Schema;
use Oro\Bundle\EntityBundle\EntityConfig\DatagridScope;
use Oro\Bundle\EntityExtendBundle\EntityConfig\ExtendScope;
use Oro\Bundle\EntityExtendBundle\Migration\Extension\ExtendExtension;
use Oro\Bundle\EntityExtendBundle\Migration\Extension\ExtendExtensionAwareInterface;
use Oro\Bundle\MigrationBundle\Migration\QueryBag;
use Oro\Bundle\MigrationBundle\Migration\Migration;
class IbnabCustomBundle implements Migration, ExtendExtensionAwareInterface
{
    /**
     * @var ExtendExtension
     */
    protected $extendExtension;</p>
 
<p>    /**
     * {@inheritdoc}
     */
    public function setExtendExtension(ExtendExtension $extendExtension)
    {
        $this->extendExtension = $extendExtension;
    }
    /**
     * {@inheritdoc}
     */
    public function up(Schema $schema, QueryBag $queries)
    {
        /** Tables generation **/</p>
 
<p>        $this->addCustomSourceCheckoutSource($schema);
    }
    protected function addCustomSourceCheckoutSource(Schema $schema)
    {
        if (class_exists('Oro\Bundle\CheckoutBundle\Entity\CheckoutSource')) {
            $this->extendExtension->addManyToOneRelation(
                $schema,
                'oro_checkout_source',
                'customSource',
                'ibnab_custom_source',
                'id',
                [
                    'entity' => ['label' => 'ibnab.ordertemplate.entity_label'],
                    'extend' => [
                        'is_extend' => true,
                        'owner' => ExtendScope::OWNER_CUSTOM
                    ],
                    'datagrid' => [
                        'is_visible' => DatagridScope::IS_VISIBLE_FALSE,
                    ],
                    'form' => [
                        'is_enabled' => false
                    ],
                    'view' => ['is_displayable' => false],
                    'merge' => ['display' => false],
                    'dataaudit' => ['auditable' => false]
                ]
            );
        }
    }</p>
 
<p>} 
 

As CheckoutSource is an extend entity we have add custom relation with  Oro\Bundle\EntityExtendBundle\Migration\Extension\ExtendExtension  to your CustomSource entity, now oroplatfrom will add addCustomSource function to CheckoutSource by configuration inside cache .

You can explore manyToOne  relation details inside oro_entity_config by simple select sql query and filter by class_name=’Oro\Bundle\CheckoutBundle\Entity\CheckoutSource’ so you will get a result just unserialize content of field data by :
$data = $this->connection->convertToPHPValue($row['data'], Type::TARRAY);
The result of extraction from full result: 

      ["manyToOne|Oro\Bundle\CheckoutBundle\Entity\CheckoutSource|Ibnab\Bundle\CustomBundle\Entity\CustomSource|customSource"]=>
      array(5) {
        ["field_id"]=>
        object(Oro\Bundle\EntityConfigBundle\Config\Id\FieldConfigId)#41948 (4) {
          ["scope":"Oro\Bundle\EntityConfigBundle\Config\Id\FieldConfigId":private]=>
          string(6) "extend"
          ["className":"Oro\Bundle\EntityConfigBundle\Config\Id\FieldConfigId":private]=>
          string(47) "Oro\Bundle\CheckoutBundle\Entity\CheckoutSource"
          ["fieldName":"Oro\Bundle\EntityConfigBundle\Config\Id\FieldConfigId":private]=>
          string(13) "orderTemplate"
          ["fieldType":"Oro\Bundle\EntityConfigBundle\Config\Id\FieldConfigId":private]=>
          string(9) "manyToOne"
        }
        ["owner"]=>
        bool(true)
        ["target_entity"]=>
        string(53) "Ibnab\Bundle\CustomBundle\Entity\CustomSource"
        ["target_field_id"]=>
        bool(false)
        ["nullable"]=>
        bool(true)
      }
    }
    ["schema"]=>
    array(10) {
      ["class"]=>
      string(47) "Oro\Bundle\CheckoutBundle\Entity\CheckoutSource"
      ["entity"]=>
      string(49) "Extend\Entity\EX_OroCheckoutBundle_CheckoutSource"
      ["type"]=>
      string(6) "Extend"
      ["property"]=>
      array(1) {
        ["serialized_data"]=>
        array(0) {
        }
      }
      ["relation"]=>
      array(4) {
        ["order"]=>
        array(0) {
        }
        ["quoteDemand"]=>
        array(0) {
        }
        ["shoppingList"]=>
        array(0) {
        }
        ["customSource"]=>
        array(0) {
        }
      }
 

You can create your Entity CustomSource Based on Oro\Bundle\ShoppingListBundle\Entity\ShoppingList  , should has relation  OneToMany with your LineItem which you can create for example based on Oro\Bundle\ShoppingListBundle\Entity\LineItem .

So now is the time to create converter of LineItem for your CustomSource to Checkout with service tagged as oro.checkout.line_item.converter example:

 
    ibnab_custom_source.line_item.converter.order_template:
        class: 'Ibnab\Bundle\CustomBundle\Converter\CustomSourceLineItemConverter'
        tags:
            - { name: oro.checkout.line_item.converter, alias: custom_source }
 

So now you can create an action to create order from your custom source and example inside src/Ibnab/Bundle/CustomBundle/Resources/config/oro/actions.yml:

 
action_groups:
    start_custom_source_checkout:
        parameters:
            customSource:
                type:  Ibnab\Bundle\CustomBundle\Entity\CustomSource
        actions:
            - '@run_action_group':
                action_group: start_checkout
                parameters_mapping:
                    sourceCriteria:
                        orderTemplate: $.customSource
                    settings:
                        allow_manual_source_remove: false
                        remove_source: false
                    showErrors: true
                    validateOnStartCheckout: true
                    force: true
                    forceStartCheckout: true
                results:
                    redirectUrl: $.redirectUrl
                    checkout: $.checkout
                    errors: $.errors
 

IS done is a big points of how to create your custom source to start checkout .