The Rig tab allows you to assign or create an avatar definition to your imported skinned model so that you can animate it - see Asset Preparation and Import
If you have a humanoid character (ie, two legs, two arms and a head) then choose Humanoid and ‘Create from this model’. An Avatar will be created to best match the bone hierarchy - see Avatar Creation and Setup or you can pick an alternative avatar Definition that has already been set up.
If you have a non humanoid character e.g. a quadruped, or any animatable entity that you wish to use with Mecanim choose Generic. After choosing you will then need to identify a bone in the drop down to use as the root node.
Choose legacy if you wish to use the legacy animation system and import and use animations as with 3.x
|Animation Type||The type of animation.|
|Generic||Generic Mecanim animation|
|Humanoid||Humanoid Mecanim animation system|
|Avatar Definition||Where to get the Avatar definition|
|Create from this model||The Avatar should be based on this model|
|Copy from other Avatar||Point to an Avatar config set up on another model.|
|Configure…||Go to the Avatar configuration|
|Optimize Game Object||When turned on, the game object transform hierarchy of the imported character will be removed and stored in the Avatar and Animator component. The SkinnedMeshRenderers of the character will then directly use the Mecanim internal skeleton. This option improves the performance of the animated characters. You should turn it on for the final product. In optimized mode skinned mesh matrix extraction is also multithreaded.|