They are used when you want to eliminate some values of a dimension when doing analysis, remove outlier, remove some value which might cause data skewed. Such as: in stock trading, I have 1000 stocks belong to 3 markets, in the analyzed date, there are some stocks got a unusual trading, then I remove them when doing calculation by 2 steps:
1. Select those stocks which you want to excluded them out before doing calculation
2. Select alternative/ excluded -> all deselected values (or Not available if you use excluded) values will become selected.