The default values are usually comes from V_T539J there you can define the wage types and how to evaluate i.e which wage type values have to be consider for the evaluation.
Let say XXXX wage type has to be evaluate 30% on some YYYY component so you can define that in V_T539J so that value will be defaulted against to the XXXX component which is equal to 30% of YYYY and then system indicates that value with "I" when ever some one changes the defaulted value manually then system removes the indicator "I" i.e defaulted is 10 now changed it to 11 then "I" will be removed.
If you wanted to give provision to do manual changes then you have to enable the indicator Re writable in V_T511 against wage type.
you can define module variants as well in same table as per requirement.
Regards
Venkatesh