A few more notes, on Schengen zone airports...

Schengen makes life even more interesting. Some of the key rules:

- Intra-Schengen flights don't have routine immigration checks. However, these can be reinstated on request, by any national government in the scheme.
- Airports in the Schengen zone can (but don't have to) consider intra-Schengen flights to be sterile. Extra-Schengen flights are considered to be non-sterile.
- The Schengen zone and the EU are not one and the same. Some EU countries are non-Schengen and some Schengen countries are non-EU. So customs and immigrations requirements don't match.

It makes life complicated for airport authorities, particularly those with restricted space. Many big, Schengen-zone airports are there mostly for transit traffic (e.g. AMS) and so have been built with transfer and free movement in mind. Others have huge volumes, but are space-restricted, and need the flexibility to move aircraft from non-Schengen on to Schengen flights without having to physically move the aircraft. Some have been able to separate Schengen and non-Schengen zones. Some haven't. Some separate long-haul from short-haul, some don't. So, you may see:

- Intra-Schengen areas with a central security point for entrance and transfers and freely mixing inbound and outbound passengers (e.g. AMS).
- A central security point for non-Schengen, but inbound and outbound passengers segregated.
- Non-Schengen inbound and outbound passengers mixing, but security is located at each gate, so every passenger must clear it before boarding (even those originating within the Schengen zone).
- Domestic may be lumped in with intra-Schengen. Or it may be entirely separate, due to customs regulations (e.g. OSL).
- Gates may be flexible between intra-Schengen and extra-Schengen. If security is always at the gate, this may be achieved by having an optional per-gate immigration check (e.g. VIE). If security is central, this may be by taking the multi level design a step further: level 0 for non-Schengen arrivals, level 1 for Schengen departures and arrivals, level 2 for non-Schengen departures (e.g. FRA, AMS).