For rear cog removal for either freehub or cassette cogs see Cog Removal
A freewheel is considered a thread-on ratcheting mechanism. The cassette cogs use a freehub, where the ratchet stays with the hub. The FR-1 is intended for the freewheel (older or less expensive type). The FR-5 is for the cassette cog lockring.
A typical cassette lockring is below, it uses the FR-5.