I've seen those pages many times. It probably well understood for an experienced asterisk hacker but not for a somewhat beginner.
ucm6100 example is given to by grandstream support when I asked them if there is any detail information on using ht503 FXO interface with asterisk. On this very forum there were people who used ht503 as far as 2008 but really none posted detailed steps, just pieces so that was pretty useless.
For outgoing, instead of long list of useless parameters provided in samples I created a CUSTOM trunk, leaving all defaults besides Trunk Name set to arbitrary name and Custom Dial String set to SIP/$OUTNUM$@ht503_ip_address
Based on the Figure 12 number of parameters (two) and the same logical meaning (name and address) it is an exact answer on my original question.