The Blend Object constraint will constrain the blending of a child object to a specific parent. The Sequence value defines which parent the blend is constrained to, and the Rest Frame defines the frame at which the blend will reset to its default value.

This could also be useful to prevent rotations or scales in certain areas. For example, if you want to prevent the base of the spine from rotating when the hips rotate you can insert a Blend Object between the hip and spine bones. Then, in the Blend section of the Parents tab, toggle the three rotate buttons (RX, RY, RZ). Other uses for this tool include blending between two sets of controllers, such as arm space or neck and spine.

The Blend object can also be used to inherit translates and rotates from two different objects.

The Object CHOP is used when you want to constrain two objects in different hierarchies.

You can use hscript expressions such as vtorigin, or python expressions such as mlookat and mlookatup.