What is the exact meaning of "On Commit" in ISC DHCP server configuration?

I know there are three kinds of events we can use for scripting in ISC dhcpd -- commit, expire and lease. My problem is, I am having a little problem in understanding the "commit" event. I know "commit" event means when the server has made a commitment of a certain lease to a client. But it would be easier to understand this in DORA context.

That means, when the "commit" event will trigger in ISC DHCP server? Is it when the dhcpd is ready to offer a lease to a client? or after it has leased IP to a client?

For example, in the following configuration -

on commit {
    set ClientIP = binary-to-ascii(10, 8, ".", leased-address);
    set ClientMac = binary-to-ascii(16, 8, ":", substring(hardware, 1, 6));
    execute("/usr/sbin/test.py", "commit", ClientIP, ClientMac);
}

when exactly will the test.py script will execute? Before the DHCP ACK packet exits from DHCP server or after DHCP OFFER packet exits from DHCP server?

Thanks in advance. Cheers :)