The Power Fit Index (PFI) indicates the overall level of gradual pressure distribution woven into each pair of BootDoc's Power Fit Socks. PFI 30 designates a gentle pressure distribution while PFI 90 classifies as strong and is therefore only found in BootDoc's performance series.
Over the years, I have tried various devices to keep my feet warm during outdoor activities in NH in the winter. I like the socks because I can use them in any of my boots, so I bought a spare pair so I can wash one and wear one. Changing the heat setting on the battery pack isn't easy if you are wearing layers, so I usually set it to Level 2 for most conditions.