The Physique modifier has five different sub-object levels. Each kind of sub-object has various controls.

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Envelope
Envelopes define a link's area of influence on mesh vertices.
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Link
Links are the segments of the Physique deformation spline. Link sub-object controls adjust the way link movement affects the mesh.
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Bulge
Bulge controls are for creating flexing muscles.
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Tendons
Tendons provide a way for multiple links to affect an area of the mesh.
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Vertex
For Physique, Vertex controls let you fine-tune the influence of envelopes.
Physique Selection Status Rollout

This rollout appears for all of the Physique sub-object levels. For all levels except Vertex, it shows the name of the hierarchy object associated with the active link. For the Vertex sub-object level, it prompts you to “Select Vertices” unless Select By Link or Assign To Link is active, in which case it displays the name of the hierarchy object associated with the active link, as it does at other sub-object levels.