I’m not an Elecraft owner, so someone could come along with better info. But here is what I find reasonably easily about the installed options.
The Elecraft K3 amateur radio transceivers are generally popular with amateur operators who participate in contests. Elecraft has a legacy of being a kit radio company, and that still shows to an extent on the K3, in that while no true solder it up yourself version exists as pretty much every board has surface mount components, you could still buy it as a chassis and a set of boards to install. And the features could be fine tuned by which boards you elected to order and install, and what upgrades you put in later. That said, some of these cards might even be in the base spec, and simply on cards to allow later redesign and upgrade, like the KSYN3 card. If you are trying to sort out value, (and if my memory is correct) one if the bigger questions is if it only has the base 10W RF power amplifier, or if it has the 100W RF power amplifier that is pretty standard in all its competitors.
KBPF3 General receive card - this allows the unit to listen to signals outside the amateur radio bands
Filters 2.8kHz, 6kHz, and FM IF filters installed. These allow for different widths of RF signals to be passed to the demodulator, based on what type of signal is being received, and how much adjacent signal you are trying to reject.
KRMXR, is an RF mixer, it looks like it is usually associated with the noise blanker card. These radios are sometimes equipped for Morse code only, so this could be a required upgrade for Single Side Band voice operation, or it might have been in the base spec.
The K3REF appears to be standard crystal frequency, and appears to be part of the base spec.
The KSYN3 rev A is the original frequency synthesizer card. The KSYN3A is a newer one that is more desirable.
