A vary of sound effects can be attached to the armrest by a plastic (nylon) restraint bracket or strap.
You could also use a strong double sided type to secure it from scratching and shaking.
The two grooves on the armrest support the straps from sliding sideways while the smaller "horn shape shelf" maintain
a balanced support to the effect base.
The same armrest module can be attached along the profile in different alignment (as shown in figure A) as single or couple
In some alignment two or more armrest can create a curvature silhouette that may support the musicians body
(arm and thigh) while playing the instrument.
Different bodies (hollow or solid) can be attached to The profile by using screws and slot nuts along the profile grooves.
You can find out more about this feature at it's project page