hi Nick, based on your question, I think the best thing for you to do would be to bookmark and read this page top to bottom, particularly the syntax section and concentrate on the table:
https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FRe...
The table gives you a concrete example of the difference between how you would show this in reporting vs in a calculated field. I'm personally not aware of you needing to use "DE" for calculated fields at the moment, but I also don't think it hurts anything, so I'll leave it to you to test out.
One thing I'll also mention as you start your journey to understanding syntax: in a month or two, this will all change (for release 22.3 they are planning to change the syntax for custom calculated fields). I'm a bit sad as I think it may render a few posts in the community useless but I also think it's an important change that will make things easier for new system admins to learn syntax.