Any break-up of the Euro would have to be done in an instant with each country reverting to its own currency with its own exchange rate which floats against all the other European currencies..
These letters identify which country issued which Euro note and so indicate what that note will be worth in the event of a break-up and a reversion to the old currencies.
Lucky old you if you have euros with an X prefix because this means they are German-issued euros and will rise in value against all other European currencies when Germany reverts to DMarks.
Poor old you if your euros have a Y prefix because these are issued by Greece and will become lowly drachmas - sinking against all other currencies when the break-up occurs.
P is good - that means Holland-issued euros - good and strong; G is Cyprus - dodgy; V is Spain - also dodgy; T is Ireland - very dodgy; M is Portugal - another dodgy one; S is Italy - problematic; Z is Belgium; N is Austria; L is Finland, F is Malta; D is Estonia; E is Slovakia; U is France which is anyone's guess.
So next time you're in the Bureau de Change picking up your euros - check that the letter on the serial number is a solid German X.