input parameters for gap_fit

[netscape88]

Dear developers,

I am learning how to use the GAP code and confused by some input parameters.

1. From the literature (PRB 87, 184115, 2013), SOAP is the most sophisticated descriptor. However, from some examples provided by the website (https://libatoms.github.io/GAP/gap_si_surface.html), two additional descriptors (distance_2b and distance_3b) are used. I am wondering why it needs three descriptors for fitting. Is the distance_2b/3b descriptor specifically useful to deal with specially situations that SOAP cannot do?
2. Different kernels are used for differnt descriptor (ARD_SE for distance_2b/3b and dot_product for SOAP). Is there any rule to choose a "right" kernel for a descriptor？
3. For distance_2b and 3b, they all use the "delta" parameter, but why they are so different (1.0 for 2b and 0.004 for 3b)? Is there any rule to setup delta and theta_uniform?
4. For SOAP, how to reasonable setup the parameters of zeta, delta and atom_sigma?

Thank you very much
Yongsheng