Time steps in Explicit Dynamics Analysis Using Altair Radioss?

Hi ,
I am just started the Radioss solver using Radioss Elearning by Altair One.
I have little bit confusion about time stepping.
What is Critical Time Step, Element Time step & Nodal Time ?
∆𝑡 <∆𝑡c, Here what is ∆𝑡 and ∆𝑡c ?
I know ∆𝑡e is Element time step.

Thanks & Regards,
Sivaprakash V.