When you need to reset vertex, envelope and other skin parameters to a Physique mesh, 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 the reinitialization mechanism to effect its inclusion. 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.