Generally:
00X-19X: trunk services. there are some patterns along the numbers, but some may get extended out of their "home" town but the number is kept, thus it's not quite as systematic. i've tried to make a rough categorization below.
20X-21X: trunk services. from services long withdrawn using this slot, it happened that most of these services are very short, point-to-point and/or specific routes, like shuttle services.
22X-38X: feeder services (mostly SBS Transit but some SMRTB due to old numbers being kept when SMRTB took over some of SBST's operations)
39X: used to be changi airport services, but now it's not being used anymore. could end up being used by feeder services.
4XX: probably something like short, point-to-point and/or specific routes of 20X-21X, but since the majority of this slot is empty (only 400, 401, 403 & 410), it could end up for other uses.
50X-51X: SBS Transit Express routes.
52X-53X: SMRT Bus (Tibs) Express routes. no routes now.
54X: Bus-Plus routes.
551-554: CityLink Direct routes. no more in service i think.
555-559: some bus company that provides premier services between certain private estates to the CBD
571-579: special private mini-buses providing services between certain private estates to the CBD
600-619: City Shuttle Service routes
620-639: Scheme A routes?
640-659: Scheme B routes?
700-709: RapidBus routes. there is one & only one svc 700. now the scheme is defunct, but the number 700 still stays.
710-719: Jurong Island Services (JIS) routes
720-799: Not explicitly defined. Some special season-specific routes (like River Hongbao Service every CNY) that do not have a number usually uses the 79X slot to specify a number for their routes so that they can print old paper tickets that display a number for their route.
800-999: SMRT Bus (Tibs) routes
800-824: SMRTB feeder services. 80X, 81X are from Yishun; 82X were from Jln Kayu (withdrawn).
825-829: SMRTB special trunk services (or formerly such services)
830-849: SMRTB routes. Not explicitly specified yet.
850-899: SMRTB trunk services 85X are from Yishun except 858 (Woodlands - Changi Airport PTB via Yishun); 86X were from Sengkang.
900-924: SMRTB feeder services. 90X, 91X from Woodlands; 92X from Bt Panjang.
925-929: SMRTB special trunk services (or formerly such services)
930-939: SMRTB routes. Not explicitly specified yet.
940-949: SMRTB feeder services. From Bt Batok.
950-999: SMRTB trunk services. 95X were express (but charging trunk fare) services from Marsiling; 96X from Woodlands; 97X from Bt Panjang; 98X from Sembawang or Choa Chu Kang.
xxxA, xxxB, xxxW: Additional Shortworking Trips. supposed to cover only heavier load sectors of a route. some SMRTB services still use 'W' (e.g. 67W) for their suffix due to an old convention, meaning "Weekend" additional shortworking trips.
xxxE: SMRTB Intra-town service that terminates at the interchange instead of going to another part of the town. Many years ago, it also meant an alternative express route of a main service (e.g. 16E from Bt Merah to Airport via ECP instead of National Stadium & Marine Parade)
xxxM: NiteOwl night bus services by SBS Transit. used to be used by services prefixed by the alphabet 'M' e.g. M1, M2, M3 & M4. 'M' in Roman numerals represents the thousand.
Trunk services (roughly, there are exceptions here & there)
00X-01X: historically, routes originating from the Changi/Chai Chee areas, but have since been branched out to various towns in the east.
02X-04X: routes originating from Bedok/Tampines
05X: Bishan trunk services (except 51)
06X-07X: mixed-up
08X: historically, routes originating from Punggol/Hougang areas
09X: routes originating from/in the western areas
10X: historically, Serangoon trunk services
11X: historically, Hougang trunk services
12X: historically, St. Michael's trunk services
13X: historically, Ang Mo Kio trunk services
14X-15X: historically, Toa Payoh trunk services
16X: historically, Sembawang trunk services
17X-18X: historically, Lim Chu Kang/Choa Chu Kang/Bt Panjang services
19X: historically, Boon Lay/Jurong West trunk services
20X-21X: point-to-point trunk services
In the 1980s,
300-309: Blue Arrow services. the predecesor of the Express routes (501-539) used now. plied between private estates & the CBD during peak hours.
400-409, 450: Special point-to-point services. Majority plying to places of interests etc, especially those in Jurong area.
SS?: Special Shuttle services. used to connect the furthermost station completed in the MRT System to areas yet to be covered by the MRT System in the late 1980s. SS1 from Yio Chu Kang to Yishun (Loop), SS2 from Clementi to Jurong East St 31 (Loop), SS3 from Clementi to Bt Batok West, SS7 from Choa Chu Kang to Woodlands.
for history of certain routes & hence their numbering, try reading these threads:
what r the routes of the withdrawn svc.?
what r the routes of the withdrawn feeders svc.?
Eulogy of certain service routes?
History of Bus service numbers