The IMU Placer Tool (available as of OpenSim 4.2) takes an OpenSim Model and the IMU calibration data and finds the initial orientations of the IMU Frames (i.e. offsets) relative to the OpenSim body segments. The IMU Placer Tool assumes that the pose of the subject in the calibration data matches the default pose of the model. If you use a different pose, you can edit the pose of the input in the OpenSim GUI, through scripting, or in XML (see Coordinate Controls and Poses to learn how to edit the default pose through the OpenSim GUI).
You can also specify optional arguments to correct or adjust for the overall difference in the heading (forward direction) of the IMU data versus that of the OpenSim model. Typically, an OpenSim model is facing in the positive X direction of the ground Frame in the initial pose, but the base IMU (e.g., on the pelvis or torso) can have any initial heading.