Not available yet. The logic engine is built (PRs finocket #6029 and #6030, rows F-E02-M and F-E02) and the editor now has a Logic tab for it on qa, but the editor itself is switched off by default (FORMS_V2 flag, database changes the owner applies at release) and the new form renderer that applies them to a visitor's answers is off unless the NEXT_PUBLIC_FORMS_V2 flag is set to 1 at build, so the public form shows the current form. Nothing below can be used today.
What can a rule do?
Show or hide a block, make it required, or skip a page, based on earlier answers. A rule that would loop back on itself is refused.
Can a form calculate a total?
Yes: add, subtract, multiply, divide, minimum, maximum and if-then, with up to six decimal places. Money is held in whole paise so totals never drift, and a division by zero is an error, not a silent zero. A visitor cannot type over a calculated value.
Can I pre-fill an answer from the link?
A hidden field can read a value from the link (up to 500 characters, control characters removed). An answer can also be shown inside later text with a placeholder; it is text only.
What if a rule reads a question nobody answered?
It counts as not answered. A page jump that reads such a question can still show the page, so mark those questions required.