Hi Sai,
There is nothing to be confused. Your check box is of the type MKPF-WEVER, and it is linked to Dictionary. Now if you look at the Table Field WEVER, it is assigned to a domain with FIXED VALUES. THis means, the Field MKPF-WEVER cannot take anyother values other than those mentioned in the Fixed Values of the domain.
If you are using it as a check box, it assumes 'X' as value once it is checked. Since there is a link to the dictionary, this value will not be allowed.
Best case is to delcare your check box as character of length 1.
Regards,
Anish Oommen