When using a Lookup Table, if the lookup value matches the lookup match field, then the destination will be set to the output field. If there are no matches then the destination variable will be set to the default value.
If the default value is empty, then the destination will be set to an empty string. This may not be the desired output in that scenario.
Utilizing the ‘Disable Default’ option on a lookup table will prevent your destination from being set to an empty string when there is no match.
There are numerous situations in which ignoring this checkbox can change the output of your extensions. One such scenario is when there are two Lookup Table extension with the same destination. If the first one matches and the second one does not then your output will be an empty string if you do not disable the default output. Here you this would just utilize the checkbox. When the default output is disabled, and there is no match, the destination variable will be unchanged from its state prior to the extension executing.