Address Setup

You can add Custom Fields for Address in Customize/Address of the Setup section.

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Fig 1: Adding Address Custom Field

Read more on types of Custom Fields here.  Note: Sections and Page Workflow do not apply to Address fields.

Address Custom Fields  automatically appear in Accounts and Contacts.

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Fig 2: PO Code and Main Entry Custom Address Fields in Account

 If Contacts inherit Address from Accounts, then they will be pre-populated when a new Contact is created from an Account.

Read more on inheriting Address here.