Here's why:
* "K9" is a specific code: "K9" is a code used in military and police units to refer to dogs. It's likely derived from the word "canine" and has been used consistently for many years.
* No set system for other numbers: There's no standardized system for other numbers like "K6" or "K3." It's possible that some organizations or units use these codes internally, but they don't have a widespread, publicly recognized meaning.
If you're interested in specific military or police codes, you'll need to look for specific documentation or consult with experts in the relevant field.