When many people write nodes for one robot, they must agree on units and directions. ROS has written agreements called REPs; REP 103 sets units and axes.
Key idea
REP 103: use SI units: meters, seconds, kilograms, radians (and m/s, rad/s). Frames are right-handed. On a robot's body: $x$ forward, $y$ left, $z$ up. Positive rotation about $z$ is counterclockwise seen from above, so a positive turn is to the left.
Why it works
A node that publishes centimeters or degrees, while every other node expects meters and radians, gives answers off by a factor of 100 or 57. Agreeing once, in writing, avoids a whole family of bugs.
Worked example: Convert
A heading of 45° in a ROS 2 message?
- $45 \times \pi/180 \approx 0.785$.
Answer: About 0.785 rad.
Common mistake
Publishing degrees or centimeters where every other node expects radians and meters.