I am comparing the linked materials attached below.

https://support.zemax.com/hc/en-us/articles/1500005490261-How-to-model-laser-beam-propagation-in-OpticStudio-Part-1-Gaussian-beam-theory-and-ray-based-approach

https://support.zemax.com/hc/en-us/articles/1500005578582-How-to-model-laser-beam-propagation-in-OpticStudio-Part-2-Using-Paraxial-Gaussian-Beam-analysis-to-model-Gaussian-beam

I think the biggest setting difference between the two materials is Object Surface Thickness. So I am comparing the following two cases.

Case 1

1. Surf #0 Thickness = 0
2. Surf #1 Thickness = 20
3. …

   

    

Case 2

1. Surf #0 Thickness = 20
2. Surf #1 Thickness = 0
3. …

   

    

Analysis Type

1.   Operands
   1. RANG
   2. GBPS
   3. OPTH
2.   Tool
   1. Analyze...Gaussian Beam...Paraxial Gaussian Beam
   2. Stands Spot Diagram

As a result,
RANG, OPTH, and Standard Spot Diagrams have the same values for Cases 1 and 2.
However, the result values for GBPS and Paraxial Gaussian Beam are different.

So I wonder in which case to set Surf #0 Thickness = 0.. in which case to use Surf #0 Thickness <> 0. And I'm curious about the physical difference between the two cases.

Note that, I know that GBPS outputs beam size based on paraxial rays.