When to consider body fixed reference frame

[curiousPep]

TL;DR Summary

I am a bit confused when to use the Euler equations and consider body fixed reference frame and when to just consider a fixed reference frame

Hello I am studying mechanics and I have been reading about having the reference frame fixed at a certain point, body fixed and also the gyro equations.
I an identify the gyro case easily as I am looking for an AAC body which rotates about an axis.
I am confused about the other two cases in which types of problems I should apply a body fixed reference frame.
What I understand in 2D it's easier not to use the body fixed reference frame. However, I am not that sure for 3D cases
Can someone give me an example or an explanation, thank you.

 

[Aaron Bex]

A body fixed reference frame is used when you want to describe the motion of a rigid body. In this case, the reference frame is attached to the body and its axes are rotated with the motion of the body. For example, if you are analyzing the motion of a car, you could use a body fixed reference frame in which the x-axis is along the length of the car and the z-axis is perpendicular to the ground. Using this reference frame, you can derive the equations of motion for the car and analyze its behavior.