Square Brackets in function parameters?

Some function parameters (e. g. Graphics.DrawPicture(Image As Picture, x As Double, y As Double [,destWidth As Double ] [, destHeight As Double ] [, sourceX As Double ] [, sourceY As Double ] [, sourceWidth As Double ] [, sourceHeight As Double]) contain square brackets.

What is the meaning of these brackets and what is the usage? I cannot find any definition of this ‘feature’. (I just know hat they work)

They are optional

Thanks

The information was here:

http://docs.xojo.com/Introduction

To get it, in docs.xojo.com, I typed Brackets in the search field and looked at the appropriate entry in the provided answers.

Nota: this is common to all development environments.