You can learn more about MLMs materials here.

You can mix and match these firmness options on toys however you please!
Unlike other stores, MLM will offer split firmness's on different parts of the toy if physically possible. This includes parts such as the Tip, Shaft, Knot, Base and Monsters Love Machines is even able to make small details such as spines and tendrils a softer or harder firmness if desired! These options are only available for Custom Complex Orders.

Monsters Love Machines comes with three standard firmness options:

Also known as 'H' (Around a 8 shore A firmness)
The hardest firmness offered currently by MLM and best to be used in specific places on the toy (such as the base) or for more advanced users. This firmness option will make detailed toys textures very enhanced and easy to feel so be sure to use plenty of lube! 
Hard has extremely minimal tacky or sticky feeling to its surface- unlike toys made of glass or plastic. This is the same as all silicone products and does not affect use of the toy in anyway. We preface this for customers who have never touched or felt Platinum Silicone products before (If this is you, we offer a Firmness Sample Set here and Silicone Squishies here which would be encouraged to explore before purchasing full size toys).

Also known as 'M' (Around a 5 shore A firmness)
Not too Soft and not too Firm, Medium firmness is a happy middle ground that has a bit more push and makes details much more defined in use without being quite so rough.
Medium can feel minimally tacky on it’s surface. This does not effect the use of the toy.
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Also known as 'S' (Around a 00-30 firmness)
With a lot of give and squish, Soft firmness is perfect for people starting out with toys or who want toys with lots of detail, but don't want it to be too abrasive. Soft is a plush option and may not be able to stand up on their own, even with large bases.
Soft has a slight tacky feeling on it’s surface. This does not effect the use of the toy.

Other firmness options:

Outside of Standard firmness options, MLM can offer Extra Soft products for those who request them. We understand some customers have specific preferences and we wish to be able to cater to a wider audience.
Due to the fidelity of Extra Soft products and the extra time that must go into their production, they incur a cost on top of base price and come with a disclaimer of use due to how easily damaged they can be when not handled with specific care.

Also known as 'S+' (Around 00-20 firmness)
This firmness is usually used for penetrable toys rather than insertable products and is extremely squishy and plushy. It will retain it's shape well and have a floppy feeling to it.
Extra soft has a somewhat tacky feeling on it’s surface. This does not effect the use of the toy.

Also known as 'S++' (Around 00-15 firmness)
This firmness is one step further than S+ and is extremely floppy and squishy. Toys are generally more vulnerable to damage in this firmness from extreme pressure or pulling.
Sumpreme Soft has a very tacky feeling on it’s surface. This does not effect the use of the toy.

Also known as 'UltraS' (Around 00-10 firmness)
This firmness is the same consistency as large puffed marshmallows and is best used in specific places, such as a squishy core for a toy. It is all give and has very little rigidity. Anything cast entirely in Ultra Instinct Soft will not hold its shape even when lightly touched or poked. It is the highest level of soft offered, it will only be attempted under strict conditions and with a preface about the safety of using such a toy— bases are meaningless in this firmness, they will just slide right up making them unsafe for certain uses. They are also more prone to being damaged by such things as sharp finger nails. This is a risk customers take themselves when ordering toys in this firmness.
Ultra Instinct Soft is tacky and sticky feeling all over its surface. This does not affect the use of the toy but does feel like touching a big honking marshmallow.