The Combine functions are useful if you are sure you will never need to adjust subdesigns again, or to enable mirroring of a selection or group of embroideries. Otherwise, it is usually recommended to use the Combine Options when you Export 
your project to an embroidery file for stitching out.
Decorations cannot be combined with other embroideries; instead they are added as a subdesign, last in the stitchout order.