DHCP Scope question?

i have a server 2003 environment set up. 15 remote offices, i was wondering how a computer knows from which scope to request an IP address from.

I was under the original impression that this was done through sites and servies...setting up subnets and such. Now that i look at my network, subnets arent set up though sites and services

???

Subnets ARE supposed to be at least recorded through sites and services.

But a client doesn't know that. A client picks up the first DHCP address it receives.

but how does the DHCP server know which scope to release an IP address from.

From further research i read that some routers, switches, relay agents...etc, track where the signal is coming from to see what subnet it originated from...and then the dhcp server recieves this new information

The DHCP server looks at what subnet the DHCP request comes from, and replies with an address in that subnet.

whats the point of subnetting through sites and services then?? To control replication?