Social casual events could be a birthday party, dinner at a friends place, etc… and business casual will be events that you attend with business colleagues, like an after office party or in some cases it can be a friday at work or in more relaxed office environment it could be a regular day at work.
Social events could have fitted jerseys, add a lot of colour to it, becauee its a social event, the impression or image to be created is not of a serious worker but of fun and friendly nature. You can also experiment with fun shoes.
With business causal, although it is casual, it is still a buisness day or a buisness event. Your collegues will be there with you and although it does not mean that you have to be very formally dressed always but conservative dressing works when you are trying to create an image amongst office crowd. Opt for more classic cuts and styles, you can wear polo tees or half sleeves shirt, or add some quirky print in small size or few colours to create your own unique personal style.