So I sometimes hear people refer to dress shirts and then business casual shirts separately. Can't you wear dress shirts for business casual, just unbutton the top button and don't wear a tie?
Also are button down collars acceptable for business casual?
dress shirt vs casual shirt
The distinction here is being drawn between two different types of button down shirts. The sports shirt and the dress shirt. Here's an excerpt from Lawrence Hunt fashion on the distinction between the two.
It is most commonly part of a man's business attire, and may have a variety of different collar styles, as well. Most dress shirts are long-sleeved and they have buttons down the full length. A dress shirt with a tie and suit jacket is semi-formal; a dress shirt without a tie, whether or not it is paired with a suit jacket, is considered semi-formal.
Sport Shirt or Casual Button Down
The differences between dress shirts and sport shirts certainly have some blurred lines, but there are a few simple ways to tell them apart. For example, a dress shirt is typically styled more conservatively, with stiffer collars and neutral colors. On the other hand, a sport shirt may contain bigger and bolder patterns, collars that are less stiff, and even epaulets and pockets. Some sport shirts are made from materials generally deemed "comfortable" such as denim, and still others contain blends of cotton, polyester, and other materials. Sport shirts pair well with jeans or slacks and are perfect for everyday casual wear.