One of the reasons the CBM was chosen for the resupply ships (including Dragon) is that the CBM has a larger inside diameter and so larger pieces of equipment (such as standard equipment racks) can be delivered through it, while the docking connectors cannot pass them.
All dimensions are in millimeters. All angular dimensions are in degrees
Implementors in non-metric countries, take note. Nothing is more annoying than arriving at a place where you might want to use your docking interface, only to find out it doesn't work. :)
Which has sort-of happened already in history of space exploration, when Mars Climate Orbiter mis-handled Mars orbitial insertion and was lost. The problem was traced down to mis-match of measurement units used in software (Newtons vs. Pound-force).
Ugh, we get it, everyone thinks they're being cute and its a way to mock US units. Yes, yes, this isn't getting incredibly tiresome.
"The error had occurred when a symbol was being transcribed by hand in the specification for the guidance program. The writer missed the superscript bar (or overline) in [the formula] by which was meant "the nth smoothed value of the time derivative of a radius R". Without the smoothing function indicated by the bar, the program treated normal minor variations of velocity as if they were serious, causing spurious corrections that sent the rocket off course. It was then destroyed by the Range Safety Officer."
* require substantial force and velocity for docking alignment and capture
* create critical operations
* affect structure fatigue life
* disturb zero-g environments
* difficult for small mass vehicles to use
* support a limited range of vehicle performance capabilities
So the miss you're thinking of isn't really where this is used. Also, this is an androgynous design (it can dock to itself) which makes things more complex.
(From this PDF: http://dockingstandard.nasa.gov/Meetings/TIM_%28Nov-17-2010%... )
I've never seen "K kg" stand for metric tonnes before. Two Ks of different cases both standing for "kilo-" therefore meaning "kilokilogram"? At first I thought "five kilograms is quite small for a spaceship" and then I had to re-read it. To say nothing of the fact that "K" means Kelvin, not kilo-.
On the other hand, what are the other options? "Mg" for "megagram"?
I cannot find a guideline w.r.t. Mg, but it can easily be confused with milligram and microgram. That may be the reason to use this weird construct ('ton' seems a better term, but it is ambiguous, too)