App Block ID
Most block types such as checkout upsell and custom fields have the option to specify an app block ID. By default, dynamic is selected. An app block ID allows for showing different blocks of the same type in different sections of checkout. For example: say you want to show a required checkbox (custom field) in the main content area but an optional gift note (custom field) in the sidebar, one can be set to the app block ID: 1 and the other: 2. You can think of the checkout app blocks as containers which can load in multiple blocks depending on any display rules you configured within Checkout Blocks.
Video guide
1. When setting up your block within Checkout Blocks, select an ID under App Block ID, for example 3:
2. Second, select the same app block ID when setting up the app block within Shopify's checkout editor making sure they match, for example 3:
Make sure your app block ID within Checkout Blocks is the same as within Shopify's checkout editor or the block will not show.