which two coordinate systems are being transformed by R_{Ti}

[chen4011]

Hello, I'm currently encountering an issue with rotation matrices when manually inputting measurement data.

I'd like to inquire about which two coordinate systems are being transformed by R_{Ti}?
From the section (3.4) implementation details of the paper "Real-Time Full-Body Motion Capture from Video and IMUs" referenced by the authors of the TotalCapture dataset, I understand that R_{ig} is assumed to be consistent between IMUs and aligned with the world coordinates through the global up direction and magnetic north.
Despite this, I'm still unable to grasp which two coordinate systems RTi represents the transformation matrix for. Therefore, I'd like to ask for clarification from everyone. Thank you!