You're right, menu PGC 6 is the key to go to the appropriate chapter button in the relevant chapter menu.

1. R[0] contains the played chapter number, as indicated in line 1. R[0] is the temporary register that is used for any purpose.
2. R[1] is the chapter button number 1-4. Look in the chapter menu, you'll see that R[1] is used to determine which button to highlight. You see here that R[0] (temporary register) is being used for comparison.
3. Take line 2, "if (R[0] != 1) goto line 5", for example. Just read it from left to right. Translation: if the value in register 0 is not equal to 1, then goto line 5. If it's equal, execute the next command below it, i.e. line 3. "R[1]=1" means assign 1 to register 1. "Link PGC 16" means just that - jump to PGC 16; if you press Ctrl+Space (Open Destination), it will automatically jump to PGC 16 for you.
4. R[4] is used to determine whether to jump to the chapter menu or not. If is 1, it will jump; if it is 0, it will not. This register was unused so I use it for this purpose. The key is in the menu PGC 1 (Root menu), line 4. I place this before R[15] line because R[15] is used to determine whether to resume the movie or start from the beginning, as shown in menu PGC 10, line 6.