Selecting the Checker type:

There are two main types of check types: Web and Group similarity, which will perform a deep web check, checking millions of public code repositories such as GitHub/Google Code, sites like Stackoverflow/Chegg and billions of lines of code available across the visible web. If more than two submissions are uploaded then the system will check for peer to peer similarity as well. 

The second type of check is the: Group Similarity Only, which will tell the system to only check the similarity of two or more code files. This option is only works with two or more submissions, if you have only one check you will select the first option by default. 

If your looking for external code plagiarism you should always choose the web and group similarity option. If your only looking to compare the similarity between code files you should select the Group Similarity Only option.