SVG Filter Regions confusion

The SVG 1.1 spec section 15.7.3 says this:

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All intermediate offscreens are defined to not exceed the intersection of
‘x’, ‘y’, ‘width’ and ‘height’ with the filter region. The filter region
and any of the ‘x’, ‘y’, ‘width’ and ‘height’ subregions are to be set up
such that all offscreens are made big enough to accommodate any pixels
which even partly intersect with either the filter region.
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The first sentence explicitly defines the offscreen buffer size to be the
intersection of the filter region and the filter effect subregions. The
second sentence implies to me that the offscreen buffer size must be the
union of the filter region and the filter effect subregions. I'm totally
confused.

Is this just a case of bad phrasing? In particular, the phrase "are made
big enough" could be "are big enough"  Is the intention to restrict the
valid values of the subregions such that they remain inside the filter
region?

Otherwise, what does it mean?

Stephen.