Hello
I don't know whether this is on the same lines as what you were on about, but the transition altitude differs for airfields in the UK e.g. in London TMA 6000ft, yet regional airfields 3000ft, because of SID's and STAR's - the 6000ft transition altitude allows separation of tracks and levels between the aircraft on the SID's and STAR'. Thats my understanding of the differences in them. Does that make sense, even if it could be along different lines??