Fusing absolute pose data

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The only reason to set the differential parameter to true if you only have one source of pose and/or orientation data is to ensure that the output of the state estimate starts at 0. For example, if your input pose data starts at (10, 23), then using the differential parameter will make sure the output position still starts at (0, 0). This is actually more important for orientation from IMUs, as the orientation is probably not going to align with your world frame (i.e., it will not start at 0).

In any case, the intended use for the differential parameter was to allow for multiple independent measurements of absolute pose/orientation data. I should clarify this in the tutorial.

One exception: GPS data or measurements of absolute position estimates resulting from, for example, landmark observations should also not be fused differentially.