AWD systems drive all four wheels all the time.
Most 4 WD systems can’t drive all four wheels all the time: usually dry pavement. “Automatic” 4WD systems detect wheelspin with the ABS sensors and activate the transfer case. But because they don’t - can’t - drive all wheels all the time, they are 4WD.
To accommodate drivetrain speed differences, AWD cars have some sort of center coupling.
Subaru 4 cylinder automatic cars have a viscous coupling that locks up when it detects wheelspin, so the torque is distributed 50/50. If there is no wheelspin, most of the torque is transferred to the front wheels.
Most 4 cyl manual shift Subarus have a clutch pack in the center. Torque is distributed 50/50 front/rear until wheelspin is detected. Then the clutch pack locks.
The more expensive Subarus have the VTD drive system, which has a center planetary geared differentisl that drives torque 45/55% for better handling. They can transfer torque from 90% front to 90% rear. Most but not all have a rear mechanical LSD. The original Audi Quattro system also had a center planetary differentisl, often referred to as “Torsten” which stands for “Torque Sensing.” Older Acuras also had a similar center diff.
The most expensive Subarus have a driver-controlled center differential, which allows the driver to select front/rear torque distribution, or select a “driving mode” for pavement, gravel, snow/ice, etc. They often have front and rear LSD as well.
To make matters more interesting, many Subarus outside the US come with a low range....
The manufacturers play fast and loose with these terms. A number of vehicles like the Jeep Grand Cherokee may have a 4WD or AWD system, some 4WD or AWD vehicles may have a locking center differentisl, and many 4WD or AWD vehicles also have 4 wheel Electronic Skid Control (ESC) which uses the ABS sensors to detect wheelspin and apply braking to individual wheels to control traction or turning.
Many “cute-Ute” systems are described as “AWD” because of substantial variations in the amount of torque transfer they allow: shut off above a set speed; or use clutch packs with limited duty cycles that cannot be used full-time.
