Use it for questions about the Form API. Don't use it for entity or entity field forms.

Forms are the primary method for obtaining and acting upon user input in Drupal. Module developers can use Drupal's in-built Form API to provide a familiar, uniform, and secure user interface.

The API for Drupal 7 forms can be found at Form generation; a quick start guide to creating forms can be found at Form API Quickstart Guide.

If the question is about the API that allows to create forms in Drupal, consider adding to the question.

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