How to Calculate Overhead Bytes for 1 Table

HERE why will be overhead of 7 bytes or how to calculate overhead?

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-- Create a table with 393 length + 7 bytes overhead = 400 bytes
-- Therefore 20 records can be stored on one page (8.096 / 400) = 20,24
CREATE TABLE CustomersHeap
(
    CustomerID INT NOT NULL,
    CustomerName CHAR(100) NOT NULL,
    CustomerAddress CHAR(100) NOT NULL,
    Comments CHAR(189) NOT NULL
)
 

Manish