The Codes Table - found under Settings > Advanced Settings > Developer's Corner - contains the IDs of Lists, Groups, and Recipient Fields that you will need when retrieving or updating information in MailUp via forms and/or the MailUp API.

Lists

Provides the following details about Lists that have been created in your MailUp account:

List code
List name
List Guid

Groups

Provides the following information about Groups that have been set up in your account:

List code
Group code
Group name

Data fields list

Provides information about the custom recipient fields set up in your account:

Field code
Field name

Examples of situations in which you will need to consult this page are:

Integration between MailUp and WordPress (for more information see this page and this page),
Insert a custom subscription form on your website (for more information see this page).