A structure is a collection of unique fields and field groups. Fields contain information about themselves, for example what type of field they are and any associated metadata, but no content in themselves (content is held in the item).

A structure consists of:

a unique identifier (uuid)
zero or more groups.

Each group has

it's own unique identifier (uuid)
an author-friendly name
and zero or more fields.

A field has

a uuid
a field_type (one of text, attachment, guidelines, choice_radio, choice_checkbox)
an author friendly label
as well as an optional instructions field, which may contain helpful instructions to the content author/editor as to how to fill a certain field.
metadata - used to control specific settings based on the field_type. For example
- is_plain: bool - on text fields will change the field type to a text field (default is rich text field)
- validation - may set a maximum word/character limit on a field. e.g.: validation: { rule: 'word', limit: 150 } (Note that limits are soft limits - the content editor may exceed the value but the UI will display a warning. We believe this provides a better authoring experience)
- choice_fields - only applies to choice_radio and choice_checkbox field types. May contain an array of options, where each option contains an optionId and a label. May also contain an otherOption which will allow content authors to enter a custom value (in the item) i.e. select an option not pre-defined in the options list.