There are 3 main types of wedge or push-in bulb in cars and trucks these days.
5mm wedge base called T5
9.3mm wedge called T10 or T15.
16mm wedge called 7440, 7443, 3156 or 3157 depending on the type.
T series numbering came from the diameter of the glass part of the incandescent bulb.
3156/3157: Replaces 25mm glass globe type
7440/7443: Replaces 20mm glass globe type
*** Both above lamps all have the (16mm) large wedge base.
T10 and T15
Both of these have the same 9.5mm small wedge base.
The T10 version is generally used in dashboards and other small recessed parts of the car.
They generally aren't used as reversing or parking lights.
The T15 version (which originally was a larger 15mm glass bulb) was a larger incandescent bulb and therefore as an LED equivalent we use the T15 LED lamp.
We enlarge the bulb's diameter & power for the T15 wedge.
Code #194 comes from the GE number for T10mm glass globe lamp.
Code #906, 912, 921 ...comes from the GE number for T15mm glass globe lamp.