This option is only relevant for the Embedded database.
Tables that use custom compression for compressing data need to pass through a re-compressing process from time to time to ensure that the data is being efficiently compressed. It is recommended that you rebuild custom compression tables every six months or so (if the data is changing, of course).
To initiate a table re-compression, select Recompress from the Maintenance pull down menu, select the tables that you wish to re-compress, then click the Recompress button.
The option will be presented whether to use a temporary table during compression or to "attempt in place". Using a temporary file is far safer than attempting in place, as any system failure during such an attempt will leave the table corrupted. In-place re-compression should be attempted only on tables that are currently backed up. If the selected table is not custom-compressed, an error message will report this fact.