An Aircraft will be instructed, by the comtroller to Squawk an Ident - to assist in identifying it on a radar screen.
Some Idents have specific - World-wide Functions - 7500, 7600, 7700, others are controller specific, by area.
a UK SSr list is available from the CAA website.
I don't know how the EU codes are allocated, but in the UK, I can 'SEE' if the aircraft is under Military or Civil control, and the area it is operating in.
codes around 1750-1770 are allocated, in the Borders ( England/Scotland) to the Spadeadam Electronic Warfare range - as an example.
Hope this gives some ideas.