Lets try to understand how this all words with some code.The HTML code of the page looks something like this.
From the above markup you can make out that there are four required field validators and out of the four validators three have validation group property defined. In such a scenario when you click the submit button only the validator which doesn’t have validation group will be executed i.e. RequiredFiedlValidator4 will only be executed. Since there are more than one validation group assigning button’ ValidationGroup wont work as it will execute only validator controls which belong to the assigned validaiton group. Other validator controls belonging to other validation group won’t execute. Also there is no way to specify multiple validaiton group using the ValidationGroup property of the button control.
In the above code you can see the Page_ClientValidate function is called four times, first by passing the first validation group name, second time passing on the second validation group name and so on. If you see the last call to Page_ClientValidate, it is called with an empty argument. When call Page_ClientValidate with no argume/validation group name it executes all the validation controls in the page which don’t have a validation group assigned to them. So if one wants to execute validation controls which are not having any validation group associated with them you can call the Page_ClientValidate without any parameter.
Try to know more.