Binary-decimal format means that the sign (positive or negative) is in the leftmost
bit of the field and the numeric value is in the remaining bits of the field.
Positive numbers have a zero in the sign bit; negative numbers have a one
in the sign bit and are in twos complement form. A binary field can be from
one to nine digits in length and can be defined with decimal positions. If
the length of the field is from one to four digits, the compiler assumes a
binary field length of 2 bytes. If the length of the field is from five to
nine digits, the compiler assumes a binary field length of 4 bytes. 
An item with binary-decimal format can only hold a limited range
of values. For example, a two-byte binary field with two digits and zero decimal
positions can hold values between -99 and 99. 