The value you would check for in your pre_save_post filter would be the same value you assign to “post_id” when setting up acf_form(). “new_post” is used as a default by ACF. If you use “new_post” in your acf_form() then ACF has a built in filter that runs on priority of 5 that will automatically create a new post of the post type “post”.
If you are creating a new “post” then in most cases there isn’t any reason to have a pre_save_post filter of your own.
What are you doing in your pre_save_post filter that requires special processing?
