Some URL parameters use array format. For these, the base parameter is appended with an "[]" to indicate as much.
Example:
[code]
parameter1[]=foo&paremeter1[]=bar
[/code]
With the above, the server receives both values for parameter1, which will end up as the array [code]["foo", "bar"][/code] .
Some parameters receive these without any qualifiers such as upload files, whereas most parameters require an "array" qualifier using the array syntax (Help:Array Syntax).