Muscles are grouped into 11 anatomical regions:
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Brows
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Cheeks
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Eyeballs
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Eyelids
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Head
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Jaw
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Nose
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Throat
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Tongue
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Torso
The main significance of anatomical regions – aside from providing a convenient organizational structure for sorting muscles – is that they carry default dynamic parameters. For example, displacements of the tongue generally happen much faster than those of the head due to their relative sizes and distances traveled, and these differences are reflected in the default dynamics of the two regions. New muscles added to each region automatically inherit the default dynamics of that region.
When adding new muscles, it is important to add them to the closest matching anatomical region, so that they inherit dynamic properties suitable for the muscle.
Although some of the properties can be edited after adding them, it helps to have a good base. There are also unexposed dynamic properties that cannot be edited and so are entirely dependent on the anatomical region.
If you are adding muscles for non-human anatomy, add them to the anatomical region that has most in common with the new muscle in terms of location on the body and expected velocity.