It depends on the router, it's actually configuring the DHCP server so when a particular MAC address comes in for DHCP it assigns a fixed result. I've done it on a number of different network setups, and the naming and process isn't always the same. IP Reservation is generally the network term, but other terms may be used to describe it. One can assign addresses outside the DHCP range, or one can assign the address the device is already using. Some devices take awhile to follow an address change while others will follow it merely by unplugging their network cable or rebooting. Assigning the address they are already using avoids the need to change, but the DHCP server needs to be smart enough to avoid that address when assigning dynamically. Long ago I always kept it separate, but it seems that current setups don't get confused when fixed and dynamic overlap and it avoids having to change from where it is at the moment.