Most of the time it will make sense reserve the field's output length for those values that are to be replaced and to increase the field's output length by the number of characters needed to accommodate the descriptive text.
The line is now ready for input and an underscore has been inserted below. The length of the underscore indicates the output length of the field, that is, the number of characters reserved for field value substitution.
In the example below, the airport codes have been inserted in parentheses. An output length of three characters has been provided for the airport code. These three characters are indicated by the underline (___) and may not be overwritten by additional entries.
Example
Several descriptive texts have been inserted in the following list: