SAP has a transaction STVARV to maintain your own selection variables for report variants.
This means you can have named variables for your saved variants so if a set of common parameters or selection options change you do not have to change a bunch of variants you just maintain the named variable in table TVARV.
Where do you apply selection variables in variants?
Whenever you create a variant you see screen with a matrix of check boxes in rows next to your Selection screen field names, if you check the box in the 'L' column that corresponds to the field you want to default and the click the 'Selection variables' button you get a screen where you can apply the TVARV defined values.
