Maybe I can help.
Example b. has "It" as its subject. "It" stands for "to help out":
b. It is very kind of you to help out.
=>To help out is very kind of you.
Below, in a., the subject "You" is topic, whereas in b. "to help out" is topic:
a. You are very kind to help out.
b. To help out is very kind of you.
Hope that helps.
- For Teachers