Custom Fields in Custom Reports

When creating a custom report, one of the criteria that can be used (such as marital status, etc) is one titled 'custom field'. Where do you define data that goes into the custom field?

Thank you.

Custom fields are set up in Edit > System Options > Members > Custom Fields. There are several threads in the forum where others describe how they have used custom fields.

There is also a wiki article that describes how to define and report on custom fields: Custom fields (MLS).