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You should give further explanation if you want to have greater opportunities of other users to answer, but it looks to me more an opencv2 issue than ROS so the tittle of your question is not well formulated.

From here https://www.myzhar.com/blog/tutorials/tutorial-opencv-ball-tracker-using-kalman-filter/ and from the fact that you are using openCV it is likely that units are related to pixels. On the attached link it is stated that speed units are pixels per second, google further into the OpenCV version API documentation you are using in order to verify this.