What do used depends on why you want to dynamically populate the field and what you want to use the value for.
Do you want to set the default value of the field or is there some other reason you want to set the value? Do you want it to change every time a post is edited or is this something that you want to use on the front end?
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users