1. Is it mandatory or good to keep this?
--> Think about a case: There is a sales but type of sale is missing(product) and you got a value for$5, report show as
if you are not going with unspecified you wont see this record in the report.
By default all wids in fact table are with 0
2. Can it be customized?
Looks like something wrong, the unspecified values are populated from list_of_val or LOV table.
might be new columns are added in table but not updated unspecified mappings.