What's this column [Bmk1002] in the table scan operator of my execution plan?

I have a simple script.

IF OBJECT_ID('dbo.Customers', 'U') IS NOT NULL
    DROP TABLE dbo.Customers;
GO
CREATE TABLE dbo.Customers
(
 custid INT NOT NULL,
 companyname VARCHAR(125) NOT NULL,
 phone VARCHAR(120) NOT NULL,
 address VARCHAR(150) NOT NULL
);
INSERT  INTO dbo.Customers
        (custid, companyname, phone, address)
VALUES  (1, 'cust 1', '(111) 111-1111', 'address 1'),
        (2, 'cust 2', '(222) 222-2222', 'address 2'),
        (3, 'cust 3', '(333) 333-3333', 'address 3'),
        (4, 'cust 4', '(444) 444-4444', 'address 4'),
        (5, 'cust 5', '(555) 555-5555', 'address 5');
IF OBJECT_ID('dbo.CustomersStage', 'U') IS NOT NULL
    DROP TABLE dbo.CustomersStage;
GO
CREATE TABLE dbo.CustomersStage
(
 custid INT NOT NULL,
 companyname VARCHAR(125) NOT NULL,
 phone VARCHAR(120) NOT NULL,
 address VARCHAR(150) NOT NULL
);
INSERT  INTO dbo.CustomersStage
        (custid, companyname, phone, address)
VALUES  (2, 'AAAAA', '(222) 222-2222', 'address 2'),
        (1, 'cust 1111111111', '(111) 111-11111111111111', 'address 111111111'),
       -- (1, 'cust 1111111112222222222', '(111) 111-1111111112222222222', 'address 1111111112222222222'),
        (3, 'cust 3', '(333) 333-3333', 'address 3'),
        (5, 'BBBBB', 'CCCCC', 'DDDDD'),
        (6, 'cust 6 (new)', '(666) 666-6666', 'address 6'),
        (7, 'cust 7 (new)', '(777) 777-7777', 'address 7');

SELECT  *
FROM    dbo.Customers;
SELECT  *
FROM    dbo.CustomersStage;

SET STATISTICS XML ON;
MERGE INTO dbo.Customers d
USING dbo.CustomersStage s
ON d.custid = s.custid
WHEN MATCHED THEN
    UPDATE SET d.companyname = s.companyname,
               d.phone = s.phone,
               d.address = s.address
WHEN NOT MATCHED THEN
    INSERT (
            custid,
            companyname,
            phone,
            address
           )
    VALUES (
            s.custid,
            s.companyname,
            s.phone,
            s.address
           );
SET STATISTICS XML OFF;

SELECT  *
FROM    dbo.Customers;

The execution plan of the MERGE statement looks like this.

You can download the execution plan here at https://drive.google.com/file/d/0B4xMAUd6DN6XdkZyTmJkdF9TY3c/view?

My question is, what exactly is this [Bmk1002]? Hope someone can help to explain it.