Initial Flashing Firmware - This is the firmware to use when original Linksys Firmware is still on the device, typically on a brand new router 'out of the box'.
Upgrade Firmware - his is the firmware to use if you had already loaded an earlier ChilliWRT version on the router or if you had OpenWRT firmware installed on the device.
In all cases: Do not forget to register the router after installation as per the Installation Guide.Otherwise we do not know it is yours.
How to install this firmware and replace the original Linksys firmware is explained in this tutorial.
Firmware for upgrade