From QB64 Wiki
A argument or parameter is information that a statement, sub, function or metacommand needs to carry out the task it has been given. Sometimes a argument can be optional and is not needed for the task to be carried out, in such case it will use a default instead of the argument when carrying out the task.
Arguments that are optional will be enclosed with [ and ] brackets in syntax descriptions.
More than one argument or parameter are separated by commas.
Example 1: The color argument in PSET is optional
Example 2: Must place a comma to seperate arguments if you use any other argument after it.
As you can see in the above example some statements have special arguments like B and BF in this case (B stands for Box and BF stands for Box Fill), if the argument isn't used then it will create a ordinary line.