Subheadings are arranged in logical hierarchical groupings or families (https://www.nlm.nih.gov/mesh/subhierarchy.html ).
For example, a search of the subheading therapeutic use in PubMed would automatically explode to retrieve citations indexed with the subheadings administration & dosage, adverse effects and poisoning. Not all subheadings are placed in these groupings (as some do not logically fit in any of the groupings).