All credits for the following go to user "vermon" who posted in this thread.
The following is a heavily amended version from his longer answer there.
Makerbot did start its hotend series using MK as iteration designator (for Mark). The first commercially available version was the [Makerbot] MK4 on the Cupcake circa 2009. MK4 was hand built with nichrome wire heaters and prone to all kinds of failures. They only worked with ABS and 3 mm filament.
Following here, Makerbot was skipped as a manufacturer and that it is the hotend we talk about. The correct nomenclature would be Makerbot Hotend MK#
MK5 was a complete redesign and had huge nozzles and a thick PTFE liner. It used 2 large power resistors in parallel for heating and was also prone to failure.
MK6 was the first hotend sold as a kit that I know of (however Repraps had also started using them probably before).
Mk6+ was the first cartridge heater hotend sold in kit form. The MK5/MK6 heater block was stainless steel, where MK6+ upgrade kit was an aluminum block that was slightly smaller, a lot lighter, and had the now standard 1/4 inch heater cartridge.
MK7 was the first hotend you folks would think looks familiar. It was the first move to 1.75 mm filament dedicated extruders. While a MK6+kit had parts to adapt to 1.75 mm filament, it never worked well at all and was really a failure.
Again, this is all Thing-O-Matic and Cupcake era. Other than a few clones, there really was no third party market in the US at this time. You either had a MakerBot or some other Reprap based kit. Wanhao, FlashForge- they didn't even exist and if they did, weren't talked about like now.
MK8 was the all new dual extruder hotend setup on the MakerBot Replicator. The cooling bar was thicker than the MK7 but the same all metal thermal barrier and supposedly, the MK8 has slightly different nozzle geometry internally and externally.
MK9 was a MK8 hotend, cooling bar, thermal barrier, and nozzle, but the new feeder with spring lever we know on all current models. This is why it's confusing, MK9 was a feeder upgrade, not a hotend change.
MK10 was a complete change of the hotend. MK10 uses smooth OD thermal barriers with a larger 4 mm OD 2 mm ID PTFE liner. MK10 also uses M7 threads, vs the M6 of all previous models. This is because a 4 mm PTFE liner is barely enough metal to make the outer tube with M6 threads. MK10 is completely incompatible with all previous hotend parts. Every part is different. MK10 still uses MK9 feeder parts.
MK11 on the D6 is just an MK10 nozzle and thermal barrier, but a different heater block and the cooling bar is part of the D6 central cross. Again, the only real difference is MK11 is a different heater block, and that's to adapt a different and longer heater cartridge and slide in the thermocouple. MK11 still uses MK9 plastic feeder parts.
I wanted to know to which group belongs my printer, as base is a Prusa clone with direct extrusion.
This question seem difficult to answer concretely, as the number in the series seem to be assigned based on a combination of heating block technology and format, the feeder geometry and the nozzle shape and thread that was adopted on the MakerBot printers. Your actual combination of those three may not exist in the MakerBot universe.
I suggest you read the full, unabridged version of the post linked above though, as more details relevant to your inquiry may be disclosed there.