Complex sentence: Where the image has multiple planes, these are stored by plane within row, such that all the red data for row 0 are followed by all the green data for row 0, then all the blue data, then alpha (or intensity) data.
Split into simpler sentences: Where the image has multiple planes these are stored by plane within row. Such that all the red data for row 0 is followed by all the green data for row 0, then all the blue data, then alpha (or intensity) data.