What does this error mean in english. Every where I look it is the same definition. I have created a materialzed view it gives this error. When I removed the sub-select out of the MV it works. Other wise I get this error.

So I have:

create materialized view
select a,b,c,d
from A,B,C, (select x from X) as X
where etc ....

is the issue with the (select x from X) as X? why is this so?

/*ORA-32303 mviews with user-defined types cannot reference multiple master sites
For materialized views with user-defined types, the definition query cannot reference tables from different master sites.

Do not create materialized views with user-defined types referencing multiple master sites. */

multiple master sites??? what is this?