What do the advanced settings of the Custom Code Block do?

Replace nodes” refers to using the exact value of the html node after server render and before client side JavaScript runs. This is useful when you are using server template variables (for example something like handlebars: {{product.id}}).

Scripts client only” is useful when the custom code block contains JavaScript that influences the HTML on your page, and that code should not run on the server. An example is adding JavaScript event listeners or third party code libraries.