The proper way is to use an enumeration.
I assume that you use a boolean type attribute for BTWCode.
A boolean represents true/false Yes/No and connot be changed
If you want to use different values, you can use an enumeration attribute type.
Robert is correct, the only way to put custom strings for a single boolean attribute instead of yes/no is to use an enumeration. (as far as I know)
Just for completeness I would like to point out that you can change Yes/No globally to say e.g. True/False in Project -> System texts
This will change the captions in the whole application, i.e. for all boolean attributes that you have.