About
A logical table source is a table where all its logical table sources have the same dimension combinations (domain) by logical level.
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Why
The check box “This source should be combined with other sources at this level” is a way to define that the fragment contains or not the whole domain. If you check it without fragmentation content because you know that your table doesn't contain the whole domain, you will get this warning.
Then if you have a target definition table that may not contain all dimension combinations for a specific level, you have to create an apart logical table for this table.