Essex Show Shirt Sizing?

So I've been looking at the Essex show shirts and decided that on sale, it would be a good investment... I want to have a long-sleeve show shirt, and then keep my short sleeve shirt that I have for back-up at longer shows. HOWEVER, the shirt I have is, I believe, a very large children's size (it's an 18 by RJ Classics). The adult sizes are confusing me, especially as all these different brands have different sizes.
I found this for the Essex sizes: Essex Riding Shirts Size Chart at Windgate Saddlery|Essex Show Shirt Size Chart|Essex Riding Shirts Size Charts|Essex Riding Apparel Size Charts|Essex Riding Shirt Sizes for Ladies|Essex Size Chart for Women|Essex Shirt Sizes|Size Chart for Essex Shi
But what does it mean by "Shirt Sizes" vs. "Essex Size"? Does it mean regular clothing, or the other show shirt sizing chart? Tailored Sportsman does their sizes by like 2-14 or something... is that what it is referring to?
Haha I'm so confused!

Yes, when they say shirt size they're talking normal dress sizes like if you walked into a Gap or something. I'm not sure if I agree with their chart though, it seems a touch small. Generally you go one size up from your bra size. So if you wear a 34 something? Your essex shirt size would be a 36. (Unless you're buying a TS, they are normal dress sizes.) I find that essex are a more fitted brand. If you're borderline I'd go up a size. I wear a 34 bra, dress size 4 in almost all stores (sometimes a 2 if they run small), and a 36 in essex shirts. But then again I do have wider shoulders...

Thank you so much! I think I'll go with a 38, then.