Help with creating a S2S VPN tunnel

Hello,

I have been tasked with creating a VPN tunnel between our organization and Cisco's email service. I have a form to fill out but I do not understand the difference between 2 spots on the form:

https://www.ciscofeedback.vovici.com/se/6A5348A724AA4248

What is the difference between...

Supply a network (preferably /24) that is convenient, otherwise will default to 192.168.2.0/24. Desired RFC1918 IP range (Datacenter 1, minimum /27)

and

Routed hosts/networks?

I looked at other S2S VPNs that exist on our network and it appears that some unused IP space was carved out of our 1918 space and dedicated to this connection? I assumed all it would really need was a /30 but they are requesting and we have previously dedicated more space than that to other tunnels. Why? In a frame relay, P2P or MPLS connection, all you need to allot is a /30, why does an IPSec tunnel require more?

Thank you very much :)