In Physique, when you need to reset vertex, envelope, and other skin parameters, click Reinitialize to display the Physique Initialization dialog. Using controls in this dialog, you select which category to update, and apply the new global settings you specify.

For example, if you've added a new bone to the hierarchy and want it included and influenced by the Physique modifier, you'd use reinitialization. Or maybe you've repositioned the biped structure relative to the mesh, or scaled both; you'd need to reinitialize Physique settings to recognize those changes.