Optional field, if you do not enter all test will run. If entered specify the test categories to run. You can use logical operators like:
Cateogry FilterResult
Priority1Any test with category Priority1
Priority1&SpeedTestTest that have both Priority1 and SpeedTest categories
Priority1|SpeedTestTests that have either Priority1 or SpeedTest categories
Priority1&!SpeedTestTest that have Priority1 category and also do not have SpeedTest category.

Command Line Argument: /category