Gravity Forms
+ Checkbox Tree

Some questions do not have a flat answer. Where can you volunteer: which regions, and inside them which cities? Which services do you need, and which options of each service? Which departments and which teams? In plain Gravity Forms you either show one very long checkbox list, or you build a chain of fields that appear one after another with conditional logic. The Checkbox Tree add-on puts the whole nested list into one field.

Visitors see the top level first and open the branches they care about, or type in the search box to filter the tree: matching items stay, and their branches open by themselves. Ticking a parent ticks all of its children, and a parent shows a mixed state when only some of its children are ticked. You can make parents headings only, allow only the lowest level, switch to radio mode for a single choice, and set a minimum and maximum that are checked again on the server, so a bypassed browser cannot get around them.

Build the tree in the form editor (indent and outdent with arrows), paste an indented list or "Parent > Child" paths, import a CSV, or use a WordPress taxonomy such as your post categories, read fresh every time the form is shown. The answer is saved as the chosen values with their full paths ("Europe > France > Lyon"), shown as an indented list on the entry, in notifications through merge tags and in the CSV export. It is different from the official Chained Selects add-on, which shows a chain of drop-downs with one choice per level. Everything runs on your own server, with no outside connection. For Gravity Forms 2.9 and newer.

The tree is kept in the field settings as a simple indented list (two spaces per level, with an optional stored value after a bar), so a pasted list, a CSV import and the row builder all end up in the same form you can edit later. A taxonomy tree is read from the WordPress terms each time the form is shown. On the page, the field is a standard WAI-ARIA tree: each item has its level, position and expanded and checked states, including the mixed state for partly ticked parents, and the whole tree is usable with the keyboard. The chosen values travel in one hidden input. When the form is submitted, the server checks the values again against the tree and the rules (known values only, one value in radio mode, heading-only parents, leaf-only, minimum and maximum), expands a ticked parent into its children and stores the result with full paths as part of the entry. Merge tags, conditional logic, the entry view and the export read that stored value. Everything happens on your own server — the add-on makes no outside HTTP request beyond the licence and update checks.