How does mpu6050 know where home position is?

I've been playing with a board with an mpu6050. I have it working and can get what seems to be meaningful data out of it. I am confused about how it knows it home position. I can somewhat understand that possibly gravity effects the x and y or pitch and roll but how does z or yaw know where to start?

If it has a magnetometer it might start at the current magnetic heading.

The gyro returns more or less to its home position. But that will drift.

The accelerator measures also the gravital force, so with filtering the pitch and roll is know.
A compass (magnetometer) is needed to know the yaw. That is why there is a new chip : MPU-9150. It is the same MPU-6050 plus a magnetometer.
A baromic pressure sensor can be used for the height.
A GPS is needed to know the location.

That is why you see IMU 10DOF boards: accelerometer + gyro + magnetometer + pressure sensor.