Yes, it looks that way.
Or, it may well be that the definition is "expanded" to mean volume. That is, once the vertical distance--freeboard--is measured or determined (the rpf deals with existing CBs), a volume can be calculated (the holding capacity before it reaches the flowing) for each CB.The sum of those volumes of all CBs evaluated will give you a total holding capacity of water collected before the water collected is conveyed. In a sense, freeboard although a length, it implies a volume; because that lenght may associated with 2 other dimensions of a 3D-structure (CB may be squared, rectangular or circular; freeboard in a channel, the "volume" would be width X length, and so on...)
Thank you so much for your input.