What do these numbers mean?

[Fabi_an]

Like these:
E006J4
P00214

[trezay]

That’s the pointer code, also called short code. It provides the following information about how the note was printed :

The initial letter identifies the printing facility. […] The three digits state sequential printing plates: "001", for example, would be the first printing plate created by the printer.The fifth and sixth characters, a letter followed by a number, represent the row and column, respectively, of the particular banknote on the particular plate.

In your case, E006J4 means :
E = The note was printed by Oberthur, located in Chantepie, in France.
006 = This was the 6th printing plate used by that printer
J4 = On the sheet, the note was on the tenth row of the fourth column.