Is it possible to select which fields appear in api response?

You should be able to use the $select parameter to select the fields that you want to see in the response.

This Acumatica help article shows a lot of good examples on how to set up the correct requests that you need: https://help.acumatica.com/(W(2))/Help?ScreenId=ShowWiki&pageid=1c767ad9-da6d-4047-bc93-6970ad469504

Please provide more details on the process you are trying to build.

You have the option to use $select to specifically configure what is returned, but at the same time, it is true that the data is visible to the code working with the REST API, so it is not clear why you want to limit the returned response given it is only returning the created/updated record.

The "Create Record" documentation says you can use the $select parameter

You can use the following parameters when you retrieve a record from Acumatica ERP

• $select: To specify the fields of the entity to be returned. For more information, see $select Parameter.

The response includes only the values of the fields of the created record that were specified during the creation of the record or that were specified to be returned by using the parameters of the request.

Doc here: https://help.acumatica.com/(W(3))/Help?ScreenId=ShowWiki&pageid=ecbfdb12-4853-4ff5-b8c5-eade02f9b47b