A custom field is a field you add to a GoHighLevel record beyond the built-in ones like name, email and phone. Each custom field belongs to one object (contact, opportunity, or a custom object), has a type (single line, multi line, number, monetary, dropdown, radio, checkbox, date, file upload, signature and others), sits in a folder, and gets a unique key you use in merge tags, workflows and the API.
The value is stored per record. A "Loan Amount" field holds a different number on every contact. That's the difference from a custom value, which holds one value for the whole sub-account.
In practice
You manage custom fields in the sub-account's Settings, under Custom Fields (as of October 2026). Pick the object, then add a field, choose its type and folder, and save. The object can't be changed after the field is created. Once it exists, you can put the field on forms and surveys, use it in workflow conditions, show it on the contact record and insert its merge tag into emails and SMS.
Setting up a new client often means creating 20 to 80 fields one at a time, then adding dropdown options to each choice field by hand. Fields also drift between sub-accounts as people add them ad hoc.
Where it shows up in RGS Tools
Field Finder's field browser lists every custom field with its key, ID and merge tag, and CSV export saves the list. Sub-Account Manager's bulk field creator creates many fields in one run, the dropdown option manager adds and sorts options, and the field mover and folder alphabetizer keep folders tidy.