It just makes it up
```
>>> a
masked_array(data = [1 2 -- 4 5],
             mask = [False False  True False False],
       fill_value = 12345)

>>> b
masked_array(data = [6 7 -- 9 0],
             mask = [False False  True False False],
       fill_value = 67890)

>>> ma.concatenate((a,b))
masked_array(data = [1 2 -- 4 5 6 7 -- 9 0],
             mask = [False False  True False False False False  True False False],
       fill_value = 999999)
```