RAF aircraft numbering convention?

Is there any rhyme or reason to the ID numbers allocated to RAF aircraft?

I ask because I notice in my photo archive, the picture of the C-17 delivered last year show it as ZZ171, whereas the Typhoon at Farnborough this year is ZH588.

So either the Typhoon is actually older than the C-17, which I guess it may be if it’s a developmental aircraft, or the RAF are re-using numbers.

Can anyone enlighten me please? Ta 🙂