K9 is both an ordering code as well as a designator in an image filename to indicate that the unit has the strong encryption license included. On a ASA, that means the 3DES-AES license.
If an ASA was originaly procured without the k9 code, the license can be added later (it's free). That's assuming the requester is not from a prohibited region (i.e., North Korea, Syria, Sudan, etc.) to which it is illegal for a US manufacturer to export strong encryption technology.
To form and HA failover pair, the units must be the same hardware models as shown by the "show inventory" output. They should also be runnig the same ASA software image.
