Difference between FixedTimeStep and FixedDeltaTime ?

Unity Doc:

“FixedDeltaTime :
The interval in seconds at which physics and other fixed frame rate updates (like MonoBehaviour’s FixedUpdate) are performed.”

“FixedTimeStep :
A framerate-independent interval that dictates when physics calculations and FixedUpdate() events are performed.”

Is It the same thing ?

may be it is helpful for you
https://forum.unity3d.com/threads/how-exactly-time-fixeddeltatime-time-deltatime-are-defined.322948/

Timestep runs independently of the frame rate to allow for consistent physics behavior.
Timestep default value is 0.02 (in seconds), this shows that every 20ms Physics Update will be executed. All FixedUpdate() will also be called every 20ms.

The fixedTimestep is controlling the fixed framerate.

But if your game is not heavily dependent on physics, you can always increase the TimeStep to get better results. (i.e. decrease physics calls)

(*Time.deltaTime and Time.fixedDeltaTime are the same value when called in FixedUpdate [Unity is able to tell if the current call to Time.deltaTime originated during FixedUpdate and returns Time.fixedDeltaTime])

When applying movement calculations inside FixedUpdate, you do not need to multiply your values by Time.deltaTime. This is because FixedUpdate is called on a reliable timer, independent of the frame rate.

The physics cycle may happen more than once per frame if the fixed time step is less then the actual frame update time.
You can change FixedUpdate time by calling Time.fixedDeltaTime = 0.0001f;

so, FixedDeltaTime represents fixed interval of fixedupdate.

Yes, they are the same thing. There is no difference between the “FixedTimeStep” setting in the Time manager “Time.FixedDeltaTime”. Both contain the same value which defaults to 0.02. Unlike “Time.deltaTime” Time.fixedDeltaTime can actually be set to a different value at runtime if needed. It dictates the physics framerate.