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One of the many interesting features in Drupal is the ease of changing fields in entities. The strength of admin/structure/types, admin/config/people/accounts/fields and related functionalities like Field collections make it possible for well-informed endusers to change forms easily. There are many different field types and many field types have multiple widgets to change the input and presentational behavior of these fields. And to add more even more complexity, fields can be arranged into fieldsets with multiple widgets.
This post addresses one possible way of handling this complexity.

Before custom text:

Say your client wants you to create a form to gather the favorite pets of their site's visitors. Easy enough:
my_module.module

...
...
...

/**
 * My form. 
 */
function my_module_form($form, &$form_state) {
  $form = array();

  $form['favorite_pet'] = array(
    '#type' => 'textfield',
    '#title' => t('Favorite pet'),
    '#description' => t('Please note that due to goverment regulations, we are required to report any person whose favorite pet is a Marsupial.'),
  );

  $form['actions'] = array(
    '#type' => 'actions',
  );

  $form['actions']['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
  );

  return $form;
}

...
...
...

Which ends up looking like this:

But...

Client:

"... government requirements might change, and you have a spelling mistake there! I want to be independent, how do I change the description of the field without learning how to code?"

In comes Custom Text!

You implement Custom Text's hooks, and change the form's description to use Custom Text:
my_module.module
<?php
...
...
...

/**
* Implements custom_text_group_info().
*/