Different models of Shimano SPD cleats look very similar - so much so that some users may not even realize that different models even exist! But there are currently 3 different Shimano SPD cleats on the market - the SM-SH51, the SM-SH56, and the CL-MT001, each with unique capabilities and pros/cons.
All these cleats fit the same, common "2 slot" pattern shoes. Let's review the specs and key features of the Shimano SM-SH51, SM-SH56, and CL-MT001 cleats and how they differ on the trail.
Shimano SM-SH51 SPD cleats
The SM-SH51 cleat is the closest there is to a "standard" - it comes included with most (although not entirely all) new Shimano SPD pedals. With this type of cleat, you push down and forward to clip in to the pedals, and rotate your foot outward to release ("single release" design.)
Although the SPD system is simple and easy to learn, the SM-SH51 is the "hardest" to release from the pedal, requiring a deliberate, intentional motion. As a result, this is the cleat most cyclocross, MTB, and gravel racers and enthusiast riders will want, because it's less likely to accidentally release from the pedal, like in a race-finishing sprint.
The downside of the SM-SH51 is it's a bit more challenging for new riders to release, and beginners worrying about "getting stuck" in the pedals may want the SM-SH56 instead for this reason.
Because of it's wide compatibility - almost every off-road SPD pedal (except the 858 - see below) the SM-SH51 is a great spare to keep in your toolbox in case you or a riding buddy needs a cleat to get out of a jam.
Shimano SM-SH56 SPD cleats
The SM-SH56 cleat uses the same "push down and forward" entry as the SM-SH51, but uses a different release technique that Shimano calls "multi-release".
To release from the pedal, SM-SH56 users can clip out both by twisting the foot (as with the SM-SH51) or by lifting and twisting. As a result, this cleat is a better choice for riders who are concerned about getting "stuck" in their pedals. Beginners, spin studios, and casual riders may find the multi-release design convenient.
The downside is since the SM-SH56 is easier to release when intended, it's also more likely to release unintentionally. As a result, racers and bigger, heavier, more aggressive riders sometimes prefer the SM-SH51 for this reason. Similarly, if you're an MTB'er who rides with a lot of "body english" you may find the SM-SH56 releases too easily.
If you're a cyclocross racer who sometimes has trouble releasing from the pedals for barriers or run-ups (especially when heavily fatigued), you might like to experiment with the SM-SH56 for easier, more reliable releases - just make sure you understand their limitations before doing so.
A few Shimano SPD pedal models, notably those aimed at casual and beginner riders, include the SM-SH56 cleat by default. The PD-T421 pedal, for example, uses an SPD-style clipless design on one side of the pedal, while the other side is a flat pedal with no binding system. This type of pedal is ideal for riders who switch between riding in street shoes and in cycling shoes, and is a good match for the SM-SH56 cleat.
Shimano CL-MT001 SPD cleats
The CL-MT001 is the newest Shimano SPD cleat on the market. Unlike the SM-SH51 and SM-SH56 models (with which you push down and forward to clip in) the CL-MT001 allow riders to clip in 3 different ways: traditional front first then rear (as with the SM-SH51 and SM-SH56), rear first then front, or simply stomping straight down.
This design makes the CL-MT001 appealing for mountain bikes, because it's easier to get into the pedal - for example, while starting from a dead stop on a steep hill. Reflecting this benefit, Shimano has started including the new CL-MT001 as the default cleat included with purchase of some new SPD pedals, especially those targeted for MTBs.
The CL-MT001 also has a slightly different shape than the other two cleats, which makes less noise while walking around. That's a nice bonus for bicycle commuters who might walk indoors in their cycling shoes.
Are there exceptions? Of course!
For the most part, these 3 cleat options (the 3 currently on the market) can be used with any Shimano SPD pedals. But the SPD design is decades old at this point, and there have been some outliers and one-offs over the years.
Some oddballs that you'l run into include the PD-M858 pedals and the SM-SH52 cleats. Unlike almost all the other pedal models, the M858 pedal requires the SM-SH52 cleat, specifically. Confusingly, the SM-SH52 cleat does work with some other pedals, beyond just the PD-M858! Fortunately, these are both older models that you're unlikely to encounter when shopping for new gear. If you happen to come across either, take a careful look at the tech docs and installation instructions to confirm compatibility before use.
When in doubt, Google up the Shimano tech docs and installation instructions for your specific pedal model and ensure that you use a compatible cleat for safety and proper function.