The Advanced Custom Fields plugin is a very useful plugin for creating a layout for clients to enter formatted content in the WordPress admin. The client or user will be given an interface that has rows and entry fields for the content that is required or even multiple instances of that content, like a listing of professionals with a picture and similar content.
Custom Fields, not to be confused with the “Advanced” Custom Fields plugin, is a WordPress field on a page or post that allows us (engineers) to pass variables to the page’s content. One example for using these custom fields (as seen in this image) is to pass (query_args) a post-category ID (cat=4 in the Value) so that the particular page will only show posts that are in that category (as long as the template is written to use this function). I commonly use StudioPress / Genesis themes that mostly include this kind of functionality. All I have to do is add the query_args and category ID and that page (using the Default Template) will show the posts from that category.
While working on one of our clients sites, I wanted to see what post category was being displayed on that particular page. When I went into that page, I couldn’t see the Custom Fields tab. So I went to the top-right and selected Screen Options, which is where we select a check box to make that section / tab visible. Well, that checkbox wasn’t there. After I thought a while and checked several other settings to make sure I didn’t disallow this somehow, I did a Google search and found this helpful article on StackExchange from someone who may have experienced the same thing: https://wordpress.stackexchange.com/questions/277388/how-to-fix-missing-custom-fields-after-upgrading-to-wordpress-4-8-1
After adding this code to my functions.php file, I was able to see my Custom Fields checkbox again. Thanks to Robbiegod who asked the question and thanks to Ben HartLenn who answered with this bit of code.