The 1918 Designation system.
The 1918 system was based on the meaning of names as codewords. It was used until 1927. Up to 1921 alliteration of aircraft name and manufacturer's name was obligatory; later this was often continued. The following information is from an article in Aeroplane Monthly, November 1994:


Landplanes

Fighter Zoological, Vegetable, Mineral (terrestial)

Single-seater Insects, birds and reptiles

Two-seater Mammals

Three-seater Flowers

Four-seater Shrubs

Five-seater Trees

Over five seats Metals, Rocks

Bomber Geographical (inland)

Single-Seater Italian towns

Two-seater British towns

Three-seater French towns

More than 3, under 5 ton Towns in colonies and dependencies

5 to 10 tons Towns in Asia

10 to 15 tons Towns in Africa

Heavy Armoured Machines Proper Names (male)

Under 2 tons Mythological Greek

2 to 5 tons Mythological Roman

5 to 10 tons Mythological Eastern and Egyptian

10 to 20 tons Mythological Northern Europe

Seaplanes

Fighter Zoological (marine)

Single-seater River fish

Two-seater Saltwater fish

Three-seater Shellfish

Bomber Geographical (seaboard)

Single-Seater Italian towns

Two-seater British towns

Three-seater French towns

More than 3, under 5 ton Towns in colonies and dependencies

5 to 10 tons Towns in Asia

10 to 15 tons Towns in Africa

Patrol and anti-submarine Proper Names (female)

Under 2 tons Mythological Greek

2 to 5 tons Mythological Roman

5 to 10 tons Mythological Eastern and Egyptian

10 to 20 tons Mythological Northern Europe


The 1927 RAF Naming System

Category Names beginning with...

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1. Fighters, land F

2. Fighters, fleet N (maritime significance)

3. Bombers, single-engined P

4. Bombers, multi-engined B

5. Bombers, torpedo M (maritime significance)

6. Army co-operation A

7. Spotter & reconaissance S

8. Coastal reconaissance R

9. Troop carriers C

10. Training aircraft T

11. General Purpose Aircraft G

12. Fighter Reconaissance O (maritime significance)


The 1932 RAF Naming System

1. Fighters Speed, Activity, Aggressiveness

2. Bombers

(a) Day Animals (except felidae)

(b) Army co-operation Classical names

(c) Night Inland town of the Empire, or towns

associated with the RAF.

(d) General Purpose British historical names

(e) Transport General towns and seaports

of the British Empire.

3. Flying Boats Coastal towns and seaports

of the British Empire.

4. Fleet Air Arm

(a) Fighters Mythological names

(b) Fighter reconaissance Seabirds

(c) Torpedo Bombers Oceans, seas and estuaries

(d) Spotter reconaissance Marine animals

5. Trainers Tuition, or places of education