User blog comment:Purpled/Cross-character cross-synth's legality/@comment-24785637-20150215190409/@comment-53539-20150215195352

All you need to know
 * XSY between the allocated vocaloids is fine, its just the non-allowed ones thats crossing the line.
 * XSY only works with vocals from the same Vocaloid character (example, Miku Light and Miku Vivid).
 * Yamaha never made XSY between two vocaloids possible (classic Anon and Kanon; they can't be XSY).

The point of XSY?
 * XSY is suppose to be a way of blending two tones together and combine the traits of the two vocals.
 * It gives a Vocaloid more methods of expanding their voice
 * It also realistically gives a tool to change vocals mid-song without the change noticing so much.
 * XSY works by the using a vocal as a "base" and applying the characteristics of a second vocal. If you switch these two vocals around you will get a different result.  Example; using Galaco Red as the core and Blue as the blended voice, will produce a different result to use Galaco blue as the core and blending in red.
 * This is the only way I can explain it so its kinda a crap explaination.

People have modded Vocaloid 4 to allow it to be done, this is where the "illegal" thing comes into play.

The list of vocaloids allowed to be XSY;

http://vocaloid.wikia.com/wiki/Category:XSY_capable_VB

Basically all you need to know is XSY isn't intended to combine Vocaloids with different names together, just the various vocals from a single Vocaloid.

If you don't understand it stil at this point, just remember it as IA x IA Rocks is allowed to be XSY but not Miku x IA.