Floating point operations; Exception, Flags, and Trap Handlers
Your problem is typographical. Look at a well-formatted copy and it should all make sense. So for example, the argument to the trap handler on overflow is $\hbox{round}(x 2^{-\alpha})$, where $x$ is the exact value.

The value of $\alpha$ comes from the fact that the exponent is 8 bits in binary32 (previously called "single") and 11 bits in binary64 (previously called "double"). You can think of $\alpha$ as an unsigned integer the size of the exponent field, with the two most significant bits set. Why it would be that number in particular is left as an exercise; think the possible range of an overflowing operation and what would be required to rescale it back into a representable range.