If this is a large tablespace, each cell of the map may represent hundreds of actual data blocks.
Red cells represent segments that consume a high percentage of blocks for their overall size. They may be good candidates for object rebuilds.
Toad measures Internal Fragmentation: a measure of the fragmentation of the object within the tablespace.
The actual SQL Toad uses to measure internal fragmentation can be found in the Toad FAQ, under the question "How does the Tablespace Map Work?"
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