Regarding the bottom distance of the soil layer

I want to generate API T-Z curve using soil parameters. In the PSI manual, there is a commentary about the API AXIAL STRATUM LINE. If the distance to the bottom is not blank, the t-z data is to be constant. If left blank, the t-z data will vary lineraly. How to deeply understand these two methods? And which one is better? Thank you.

I made two input files with same soil parameters but different bottom distances(one blank and the other not blank). I compared the T-Z curves of the same layers. Only the first four layers are different, and the last layers are same. I am not sure it is resonable or not.

Thank you very much.