What the others said....and here's my 2 cents.
If you're trying to find out availability for SQ flights, a lot of the availability tools out there will give you a general idea, but not the accurate results you'd get with availability on say, UA or AA.
For example, if you use the
KVS Availability Tool, you could get 2 different results for the same flight (example below, where the first was a lookup using the ITN/CWT engine and the second using the ITN/UA engine.
SQ 63 BKK 11:15 SIN 14:40 773 0 F4 A4 C4 J4 D0 Y0 B0 M0 W0 Q0 N0 V0 P4 H0 K0 Z9
SQ 63 BKK 11:15 SIN 14:40 773 0 F4 P4 A4 Z4 C3 J0 D0 S0 Y4 E0 B0 M0 W0 H4 Q0 N0 T0 V0 L0 K4
The general idea you'd get from these results is that economy is almost sold out, business is selling fast (no more discounted D seats available) and first is wide open.
However, if you went down to an SQ CTO, you could see a totally different availability on their screens. It makes a difference where you're making the booking - if you're booking an SQ flight through UA, you'd look up the availability in the ITN/UA tool....if you're booking direct with SQ, you'd have to check their availability with an agent.
P.S. I could be wrong about this, but an SQ agent told me that different SQ CTOs would show different availability for the same flight depending on point-of-sale restrictions. Don't know how that works.