(Creating bones to more easily animate complex objects or characters) While you are not required to use bones to animate with Spriter, and in act, for many types of animations bones would just be an inconvenience, for the case of animating complex objects or characters the initial investment of literally a minute or two to “rig” the character with bones will end up making work much easier and more natural, and save you a lot of time, even resulting in a superior final animation.  To create bones, simply hold the Alt key and left click and drag from the point you want the bone to begin to the point where you want the bone to end.  The point where the bone begins (the thick end) acts as the pivot point of the bone. When you let go of the left mouse button that bone is done being created and is automatically selected...if you create a new bone while the previous bone is still selected, the new bone will automatically be a child of the selected bone.  You can continue to hold Alt and create all of the bones you need for the full character...just be sure the bone you want to be the next bone's parent is selected while creating the new bone.  At any time in this process you can let go of the Alt key and select, move, rotate and scale any of the bones to perfect their position relative to the sprites you will be assigning to them.
(Assigning Sprites as children of bones) Now that your bones are set up where you want for the whole character, properly aligned with the sprites, you just need assign (child) the appropriate sprites to each of them.  To do this, simple select a bone by left clicking on it, and then hold the B key...you'll see all Sprites become more translucent.  Now if you left click on any Sprite while still holding the B key, that Sprite will become a child of the selected bone.  You will see that it is now assigned because the sprite is now more opaque.  If you click that same sprite again while holding B it will disassociate that sprite from the selected bone.  You can select as many sprites to each bone as you'd like. He sprite does not have to be touching the bone, or even be close in proximity to the bone.  Once finished assigning all sprites to their respective bones we recommend you quickly play with putting the character in extreme poses with limbs overlapping the body and the other limbs so you can double-check that all Sprites are z-ordered properly.  This way, you won't have to stop and manually fix the z-order of sprites across several key-frames once you're animating.
Creating and Assigning Bones

Spriter Pro User’s Manual version 1.4