By default blocks will show as per their eligibility defined by display rules.

If more than one block is eligible then one random block will be selected to be shown from the list of eligible blocks.

If you’d like to set a specific block to show within an instance of the app then you can set the ID of the specific block into the app’s extension settings, found in the Checkout Editor (How to add a block to checkout (Shopify Plus required)).

Then, if the set block is eligible it will appear within checkout.