Investment = Saving relation in an open economy
First it should be clear that this is an (ex post) national account equality:

$Y=C+I+G+NX$, the private saving is $S_p=Y-C-T$ and public saving is $S_g=T-G$
thus you have $S_p+S_g-I=NX$.

Later you see in this book that the net exports, which depend on the exchange rate, are exactly equal to the Net Capital Outflow. Why? because an export is like a capital import (and vice versa).