What are PDF checkbox and radio fields?
Definition
Checkbox and radio fields are the interactive PDF form fields that capture choices. A checkbox toggles one option on or off independently; radio buttons form a group where selecting one clears the others, so exactly one choice is active.
Checkbox vs. radio
Use a checkbox when each option stands alone and several can be true at once. Use radio buttons when the options are mutually exclusive and only one may be chosen. Getting the type right determines how the answers behave.
Why they're hard to detect
On a flat form, a small square or circle near a label could be a checkbox, a radio option, or just page decoration. Whether several boxes form one exclusive group depends on the layout. Reliable detection has to read both the marks and their grouping.