Timber window - extended sill

Hi all, I'm replacing a rotten timber window in a weatherboard structure with a new one ordered from a window manufacturer. The new window has arrived with what the manufacturer calls an "extended sill" to apparently accommodate the installation into weatherboard. So the bottom sill on my new window is 150mm wider each side than the rest of the window.

I have pulled out the old window and now have a hole in the wall where the old window was surrounded by the lintel, left and right jamb studs and the bottom sill trimmer. The hole allows for the recommended space and packing around the window but I'm struggling to understand how the extended sill is supposed to interface with the vertical jamb studs each side of the window. Does the sill get cut to fit around the studs so only the external portion of the extended sill remains or am I missing something obvious here?
I'll sketch it up if I'm not being clear.
Thanks in advance.

Rob


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