Airspace Structure, ATS Routes and Clearance Coordination
Structure
Airspace is divided into FIRs (with UIRs above), subdivided into control areas (CTA), control zones (CTR) and terminal areas (TMA), plus advisory routes, prohibited/restricted/danger areas and temporary segregated areas. ATS routes carry designators (A, B, G, L, M, N, P, Q, R, T, UL, UN) and are defined by significant points: VOR/NDB, named five-letter points, or RNAV waypoints.
The ICAO class table - learn it as a matrix
A: IFR only, all aircraft separated. B: IFR and VFR, all separated. C: IFR separated from IFR and VFR; VFR separated from IFR but only given traffic information about other VFR. D: IFR separated from IFR; IFR gets traffic information about VFR, VFR gets traffic information about all. E: IFR separated from IFR; VFR gets traffic information where practicable. F: advisory; separation where practicable. G: no separation - flight information and alerting only.
Coordination
Clearance coordination covers route, level, speed, SSR code and release point. A clearance is only valid once the receiving unit accepts; until then separation remains the transferring controller's responsibility. Amendments require the same readback discipline as initial clearances.
Traps
Assuming Class D separates VFR from anything - it does not. Assuming Class G means 'no service' - flight information and alerting still apply. Confusing a CTR (surface to a specified upper limit) with a TMA.
Rule of thumb
For any airspace question ask two things: who is separated from whom, and who only receives traffic information? In Class C, VFR is protected from IFR only; in Class D nobody is protected from VFR.