# API Configuration
source: https://developer.mastercard.com/mastercard-processing-debit/documentation/api-basics-section/configuration/index.md

Mastercard Processing API enables a customized configuration for each issuer to align with specific requirements. Configuration includes, among others, definitions of special fields, available characters, and dictionaries. Part of the API configuration is also a list of `operationId` enabled for each issuer. The list depends on the product that issuer wants to implement.
Note: If you try to execute the `operationId` that is disabled in your API configuration, you will receive a dedicated error. For more information about error messages, see [Error Codes](https://developer.mastercard.com/mastercard-processing-debit/documentation/code-and-formats/index.md).

## Dictionaries {#dictionaries}

API uses dictionaries to:

* Validate values used in requests.
* Transform values between API and Card Management System (CMS) in both directions.

Realizing those functionalities requires a specific configuration based on which API operation can execute validation and transformation actions. Dictionaries store the possible values of some fields used in API operations and sometimes store mappings to CMS values. All dictionaries must be configured during onboarding.

API allows limiting or extending the available characters or setting a specific mask for sending the data, such as phone number or e-mail.