: Musculoskeletal vs. muscular disorders A musculoskeletal disorder (MSD) is one that is associated with a musculoskeletal system (locomotor system). Is MSD a subset of muscular disorders or a separate pathological category

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This is more the matter of the usage of the terms rather than an actual categorization.
"Musculoskeletal disorders" is a term used in occupational medicine that refers to "minor" and mainly chronic disorders (mainly overuse injuries) that affect motion and include disorders of the muscles (chronic strain, myofascial pain), tendons (tendonitis), ligaments and joints (osteoarhritis) and nerves (herniated disc, carpal tunnel syndrome).
Examples of muscle disorders that are usually not described as musculoskeletal disorders are acute muscle injuries (muscle contusion, strain, tear).
Canadian Center of Occupational Health and Safety

WMSDs [Work-related musculoskeletal disorders] are very difficult to define within traditional disease classifications. These disorders have received many names, such as:

Repetitive motion injuries
Repetitive strain injuries
Overuse syndrome
Soft tissue disorders

EDIT:
To answer your question "Is MSD a subset of muscular disorders or a separate pathological category altogether?": Muscular disorders are actually a subcategory of "Diseases of the musculoskeletal system and connective tissue" by ICD-10 classification, but these would still not necessary be "pure" muscle disorders.
Examples of "pure muscle diseases," which mainly affect the muscle fibers or muscle function and not the tendons, ligaments or joints, could be:

Tetanus with muscle spasms (an infection by the bacterium Clostridium tetani)
Rhabdomyolysis with massive muscle fiber breakdown due to trauma, severe alcohol poisoning, adverse reaction of certain drugs, etc.

Myasthenia gravis with muscle fatigue (an autoimmune disorder) is classified as a disease of the nervous system but affects the function of the muscles.

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I only partly agree with previous answer and would like to contrast some of the points suggesting that "muskuloskeletal disorders is a term used in occupational medicine".

According to the International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) of the WHO (which is used by hospitals, health prof. and insurances to "code" a disease), there is a part called "Diseases of the musculoskeletal system and connective tissues" (Chapter 13), diseases are then categorized as follows:

Arthropathies (eg infectious arthropathies, inflammatory polyarthropathies, arthorsis)
Systemic connective tissue disorders (eg. systemic lupus erythematosus, dermatomyositis, polyarteritis nodosa)
Dorsopathies (eg spondylopathies)
Soft tissue disorders (eg infectious myopathies, mitochondrial myopathies)
Osteopathies and chondropathies (eg osteoporosis)
Other disorders of the musculoskeletal system and connective tissue

While some of the listed diseases in each of the above mentioned categories might be work related (or occupational), some may also be congenital (genetic) or acquired (drug associated, infectious, de novo, traumatic, etc...). Following link provides the extensive classification with the corresponding diseases: apps.who.int/classifications/icd10/browse/2010/en#/XIII
So MSD is a main category, which entails some subcategories as listed above. Some diseases might be "pure" muscular, other pure "skeletal" (although in most cases, this leads to muscular problems) and some might be both "muskuloskeletal". In general, the medicine speciality dealing with all these disorders is rheumatology and from my current experience, the rheumatology unit in my hospital treats a significant proportion of diseases that are not occupational related.

Edit:
Some myopathies (e.g mitochondrial) are also listed under Chapter VI, "Diseases of the nervous system" as some of them, while affecting the muscles, are also associated with neurological disorders (stroke, epilepsia).