What is mean by IMU remaining still (does gravity vector should be alligned with body z-axis while capturing values?)

[BharadwajaY]

Hi,
I have doubt on capturing the raw imu values while IMU remaining still. That means, we have to keep the IMU z-axis aligned with gravity direction. is this interpretation is correct or we can capture the values at any orientation of IMU with gravity direction.

If we can capture at any orientation, the projection of acceleration due to gravity on xy-plane will be coming as a bias in the measurements. is this interpretation is correct?