How casual should casual attire be? The best thing is to give an example.
Terms can mean different things to different people. Some terns are specific to a locale and may not be widely known to all of your invited guests.
"What does Southern Casual mean?" I've seen this question posted on wedding forums before. The best thing is to let your guests know. Guests shouldn't have to search the net to find out what a term means.
Casual dressy could mean pressed jeans to some people, and a nice dress and leisure suit to others. Does it matter? Just let your guests know if it does, and give them a range if appropriate, i.e. The reception will be at a park by the beach. Dress comfortable and wear shoes you don't mind getting sand in. Jeans are appropriate.
Another thing that will be helpful is to let guests know the setting for the wedding, and for the reception if it is at a different location. Will the wedding be indoors and the reception outdoors? Will different attire be needed. Usually for casual weddings, I don't think you would want guests to have to change, unless of course you've made it convenient for them to do so. If there is a swim party afterwords, then by all means, the guests will probably want to change and to bring their bathing suits.
The main thing to do is ask yourself some questions as if you were a guest:
What is the setting/location?
What attire will be best for the setting/location?
Is there a place to conveniently change, if needed?
I hope you have found this information helpful, and I invite your additional tips and comments.
CherylsArt at Zazzle