The numeric field "Field: xxx" was found to be using more than 8 bytes per value. Memory use would be reduced by nn bytes if this field were stored in numeric form. 

The field appears to contain only numeric data.  All numeric values can be represented in 8 bytes of storage. This field may be occupying more storage then necessary if stored as a "Dual" type.

If intentionally stored as Dual, this flag may be ignored.

If the dual storage is unnecessary, apply a numeric function to this field in the script to convert the field storage to numeric only.

QlikCookbook: Dual Storage vs Dual Behavior