Yes, it is true that EPS is the effective plastic strain. It is an element variable. It is an array.
EPST from your note is a local variable. It is not an array. It is a scalar and has a single value. See the picture below.
User Subroutine "SOLVE_STR_USER_1" is called for every element automatically by Autodyn. EPST is the Effective Plastic Strain for the current element, i.e., EPST = EPS(current element #).
EPST is used to interpolated plastic strain from the input of material hardenning curve (Yield Stress vs Plastic Strain) to obtain the Yield Stress for the current element.
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