These tapes are used to measure out logs. The end has a special hook or nail that holds the end of the tape at the end of the log, but releases when jerked; then the tape will automatically rewind.
There are a number of ways to attach a tape to a logger's belt. The device shown not only has a ring for the tape, but also holds two wedges, cutter and depth gauge files, depth gauge joiner, screwdriver/bar wrench, carburetor adjusting screwdriver, and first aid kit.
It's not unknown to use a pole for measuring logs instead of a tape. Often the ends of the pole are painted, so that it will be noticed if it gets damaged. Bucking a log a few inches short can be very expensive for the logger! Logs are usually sold by scale which typically means that they are to be cut to a nominal length in 2 foot increments plus 'trim'. Mill specifications will specify the trim which is usually 6-8 inches per log. This may seem like a lot, but the saw mill doesn't daw 40 foot lumber. Common lumber lengths are 8-10-12 foot lengths so the long log is cut into several pieces so all that trim is really needed.
The hook for the end of the tape is a sory by itself. The tape comes with a loop riveted to the end of it and with a hook attached to the loop. Naturally the hook takes a lot of abuse, so loggers have a way of making their own hook which is better than the factory hook anyway. You make the hook out of a good old fashion horseshoe nail. You bend the head end double about half an inch from the head and slip it over the loop in the end of the tape and crimp it down so it won't fall off. then you bend the pointed end ninety degrees about 5/8ths of an inch from the end (and about where the head is doubled over against the nail. Very near the end you further bend the nail to hook it slightly.
You can then stick this nail lightly in the bark of the tree and walk down the tree with your chain saw bumping knots as you go with the tape playing out behind you. When you have reached the proper spot you mark the tree with the saw, shake the tape loose so it rolls up and climb down off the log and buck it. The need to walk down the logs without falling off is why loggers have traditionally worn Calk Boots.