In general interchangeable or removable cores are a great tool for a locksmith. One of my favorite applications for IC is on panic hardware and pilfer guards. (Pilfer guards are the devices that make lots of noise when someone leaves out a door they should not have) Sometimes taking apart panic hardware is a real pain depending on the type, and the quality of the installation and who has been rekeying it. Other applications where I/C cores are frequently used are in padlocks, file cabinet/medical cart locks, cam locks, desk locks, drawer locks, key switches, and you can even see them on FedEx and UPS trucks as well as armored cars. While doing a little bit of research for this article I read that there are over 2 billion cores in use all over the world.
As for the different types of cores there are I have to start with the Small Format Interchangeable Core A/2 system. There are 3 SFIC systems A/2, A/3, and A/4. From what I understand A/3 is not used very much and A/4 is typically used in institutional locksmithting.
A/2 is hands down the most widely used system of the three, and although they all share the same hardware and keyblanks, (they also use the same keyblanks no matter if it is a 5, 6, or 7 pin core since they are all cut tip to bow) the pin kits make the difference. A/2 pinning uses a system where each pin stack must equal 23. So if you have a 3 and a 2 for your change key and master key pins, and a 10 for your control pin, your makeup pin must be an 8. I won't dive too much into the pinning here, I just want you to get the idea I'm not trying to teach how to pin them up.
So for SFIC you have your change or user keys depending on your choice of terminology, and a control or core key to remove and place the cores. The control key does not use a special blank, it is a normal blank and it is cut to the control bittings. SFIC cores can be masterkeyed the same way you would masterkey any other lock as long as you are using a 2 step progression system since there is no #1 top pin. You can also masterkey the control bittings. This is something I have done pretty frequently where the IC core its self is the lock in a new type of sliding door lock system. When the control key is used it draws the control lug into the housing of the core so that the core can be removed.
Below is a picture of an SFIC taken apart so that you can see the plug, control sleeve and housing separately.
The photo shows a Schlage Everest SFIC core that I use for one of our customers. You can see the check pin in the locked position and then again with the key inserted it is retracted.
This is a photo of my A/2 pin kit. Though this kit did not originally come equipped with nickel-silver pins that is what I stock it with now. Nickel-silver pins last much longer than brass pins.
This is the chart that shows the pin number and the pin length in thousandths of an inch.
These are my pinning tools, a Lab Annex which allows me to not only cap cores, but also dump a core into a "book" that allows me to decode the core so that I can find a core key bitting or a master key bitting. The four pronged tool dumps 4 chambers at a time, there is another for dumping only one and finally the capping tool.
This is the "book" from the Annex with a core dumped into it.
At the back of the A/2 core are two holes on either side of the keyway. Those holes are where the tailpieces interact with the core and open locks when the key is turned.
As for who makes A/2 cores? Well just like everything else in the lock world there are a bunch out there who make A/2, as you've already seen, Schlage makes A/2, there is Best of course, Falcon, Medeco Keymark, KSP, Arrow, CX5 and many others that I can not think of right now. Although they all share the A/2 system, and they all are made up going tip to bow, the only differences there really are is that some are shoulder stopped rather than tip stopped.
So, now to move on to LFIC or LFRC, (large format interchangeable cores, or large format removable cores). Among the manufacturers of these are Schlage, Mul-T-Lock, Sargent, Medeco, Yale, Corbin-Russwin and probably a few others. Mul-T-Lock uses the Schlage format which makes it easy to get hardware to support it. As for the others I don't honestly know if any of them share a format. Schlage, Mul-T-Lock and Yale use a special key to act as a control key, but Sargent, ASSA (I think) and Medeco use normal keys cut to a particular bitting to pull the control lug back.
Below are two unlabeled Sargent cores, one picked to control.
A Corbin-Russwin core that I can only pick to operate.
A Schlage LFIC core
A Schlage LFIC control key, you can see that at the tip is a flat cut to a #6 depth, this lifts a pin at the rear of the housing and when the key is turned it pulls the control lug in.
And a Mul-T-Lock core and key. The key can only be inserted one way and works the same as a Schlage.
I hope this has been helpful. This was by no means meant to be a complete course, just to introduce you to the basics.