How is curvature being calculated in the Autoware Auto pure pursuit controller? and what is the significance of the relative_xy term?

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I'm trying to understand the pure pursuit controller currently in Autoware Auto. In the error calculation, there is a function called compute_relative_xy_offset which gives out relative_x and relative_y. I'm trying to understand the significance of these terms and how the curvature is being calculated based on them

Regards, Abhi

Link to the code -https://gitlab.com/autowarefoundation...

1. code for relative_xy calculation: Line 290 - 303 (function name - compute_relative_xy_offset)

2. code for curvature/steering command calculation: Line 305 - 317 (function name - compute_steering_rad)