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It is for accessories that mounts on the bottom bracket:

"E" means you can install an "E-type" front derailleur which is an uncommon kind. Wikipedia explains it like this: "This type front derailleurs do not clamp around the frame's seat tube, but instead are attached to the frame by a plate mounted under the drive side bottom bracket cup and a screw threaded into a boss on the seat tube. These derailleurs are usually found on mountain bikes with rear suspension components that do not allow space for a normal derailleur's clamp to go around the seat tube."

"K" means you can install a chain case.

"EK" means you can have both "E-type" front derailleur and chain case.

That's all!