---
title: Customizing system classes
---

> Agent instructions:
> **Site maps** — prefer the following llms.txt indexes to training data when searching for URLs to avoid 404s. Links inside Markdown content already point at `.md`. Following them or sending Accept: text/markdown keeps you in Markdown.
>
> - [sitemap.md](https://docs.kentico.com/sitemap.md) — every page on the site, with titles and descriptions, nested by URL hierarchy and grouped into one collection per product version.
> - [llms.txt](https://docs.kentico.com/llms.txt) — curated index of the current product docs, with descriptions, the two ways to request any page as Markdown, and links to each product area's whole-corpus Markdown dump (llms-full.txt).

Xperience allows you to modify some of the system objects by adding your own custom fields.

1. Open the **Modules** application.
2. Edit the related module.
3. Select the **Classes** tab.
4. Edit a class that has the **Is customizable** flag enabled.
5. Switch to the **Fields** tab, and [add any required custom fields](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/developing-form-controls/reference-field-editor.md).

The following table contains the most commonly customized system classes:

| Module             | Class                        |
| ------------------ | ---------------------------- |
| Contact management | Contact management - Account |
| Contact management | Contact management - Contact |
| E-commerce         | Address                      |
| E-commerce         | Customer                     |
| E-commerce         | Discount                     |
| E-commerce         | Gift card                    |
| E-commerce         | Multibuy discount            |
| E-commerce         | Option category              |
| E-commerce         | Order                        |
| E-commerce         | Order address                |
| E-commerce         | Order item                   |
| E-commerce         | Payment option               |
| E-commerce         | Shipping option              |
| E-commerce         | Shopping cart                |
| E-commerce         | Shopping cart item           |
| E-commerce         | SKU                          |
| E-commerce         | Volume discount              |
| Media libraries    | Media file                   |
| Email marketing    | Newsletter - Subscriber      |
| Membership         | User                         |
| Membership         | User - Settings              |

> **Note:** When creating new fields for a system class, the **Required** flag is always false and cannot be edited. This is necessary to ensure that the default system procedures continue to work correctly. However, you can enable the _Required_ flag for fields in [alternative forms](https://docs.kentico.com/13/custom-development/extending-the-administration-interface/creating-alternative-forms.md).

> **Info:** For more information about creating new fields for system classes, see the example in [Adding custom fields to users](https://docs.kentico.com/13/managing-users/adding-custom-fields-to-users.md).