This problem analyzer is triggered when Gearset detects that your deployment will change the data type of a custom field.

Changing the type of a custom field is risky, because data might be lost as a result. Different data types also have various restrictions on how and when the type can be changed, and trying to deploy these changes can cause your deployment to fail.

Given the potential risks around changing a field's type, Gearset will warn you of the possible consequences when your deployment would make this kind of change. This prompts you to check everything's OK, and decide whether you'd like to go ahead with the deployment.