Which is the purpose of the Enable network API option ?
Enabling the network API allows anyone to retrieve a list of public books in your network via the API:
This is not always desireable (for example, Pressbooks.com has ~50,000 books so we have disabled that feature to prevent unnecessary burden on the server).
if is activate, we do allow to clone from another PB installation?
You can clone books no matter whether the network API is active or not.