Re: What is different between helper bootp and dhcp-relay?

Yes, this is how I see the features working.

Simple dhcp helper using bootp

http://www.juniper.net/techpubs/en_US/junos15.1/topics/usage-guidelines/policy-configuring-routers-or-interfaces-as-dhcp-and-bootp-relay-agents.html

Overview of the extended dhcp options.  

http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/dhcp-extended-dhcp-relay-overview.html

You can also download the full subscriber mgmt guide which contains this feature:

http://www.juniper.net/techpubs/en_US/junos15.1/information-products/pathway-pages/subscriber-access/subscriber-mgmt-provisioning.pdf