What is use of ec2-credentials table in Keystone database? The table contains following schema

| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| access    | varchar(64) | NO   | PRI | NULL    |       |
| secret    | varchar(64) | YES  |     | NULL    |       |
| user_id   | varchar(64) | YES  |     | NULL    |       |
| tenant_id | varchar(64) | YES  |     | NULL    |       |

i am not getting the purpose of this table?