What are the uses of 什会 and from which area? In restaurants in Australia we quite often have Chinese dishes which are combination for example Combination Satay or Combination Fried Rice. These contain several types of meat as opposed to something like Chicken Satay.

When this is written in Chinese on the menu this is typically written as 什会 for example with Combination Satay: 沙爹什会 Shā diē shén huì

This appears more typically with Cantonese/HK cuisine as this is more prevelant in Australia.

Is this usage a Cantonese/HK usage or does it also appear on the mainland?

Are there other uses for 什会 other than for food?
I believe the correct characters should be 什烩 and the correct pinyin should be "shí huì".
When 什 is prnounced as "shí", it means "serveral kinds of, various kinds of [something]". Generally, this meaning is used in dishes or foods, like 什锦糖(a kind of candy containing different flavors in one candy). 烩 is a method of cooking.It means,
1. After frying or sauteing the ingredients(almost cooked), braise all ingredients, usually with the starchy. OR
2.Cook different things(rice, vegetables, meat, etc.,) together with water.
So 什烩 means food cooked with the first method of "烩", with a lot of different ingredients.

Note: 
1. The character 什 really has this pronunciation, though you would only see "shén" as in the commonly used word "什么". 
2.It's difficult for me to explain something clearly related to cooking(due to lackages of words and knowledge) in English. I have searched “什烩" on Internet and found a lot of results. I am sure it should be "什烩". You might as well ask the cook to understand the EXACT meaning of 烩.