Checkout of a document
| Column | Type | References | Comments |
| id | Integer | Primary key | |
| vdb_id | Integer | ||
| created | Date | Date of the checkout | |
| document_id | Integer | db_document.id | The document for this checkout was done |
| ci_id | Integer | db_document_ci.id | Corresponding checkin ID, if null means there was no checkin |
| co_by | Integer | db_person.id | User who checked-out |
| release | Integer | Release number of this checkout | |
| revision | Integer | Revision number of this checkout |