Hi Anushka,
HFSS is an Electromagnetic Solver which solves Maxwell's equations to obtain electromagnetic solutions for a given geometry and boundary conditions. Maxwell's equations are a set of four partial differential equations that describe the behavior of electric and magnetic fields and their interactions with matter.
The surface current is mathematically extracted from the calculated H field.
You can find more information on the calculated Field Quantities of HFSS in the following link of HFSS Help,
Ansys HFSS 2023 R2 - Plotting Field Overlays
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