The auto IK option in the Sidebar enables a temporary IK constraint when posing bones. The chain acts from the tip of the selected bone to root of the upper-most parent bone. Note that this mode lacks options, and only works by applying the resulting transform to the bones in the chain.
Edit and Pose Mode
This option enables automatic mirroring of editing actions along the X axis. You can enable this option in the, while the armature is selected in Edit Mode. When you have pairs of bones of the same name with just a different "side suffix" (e.g. ".R"/".L", or "_right"/"_left" ...), once this option is enabled, each time you transform (move, rotate, scale...) a bone, its "other side" counterpart will be transformed accordingly, through a symmetry along the armature local X axis. As most rigs have at least one axis of symmetry (animals, humans, ...), it is an easy way to keep the model symmetrical.