tcp_app_win question

In kernel/Documentation/networkingip-sysctl.txt i found

tcp_app_win - INTEGER
Reserve max(window/2^tcp_app_win, mss) of window for application
buffer. Value 0 is special, it means that nothing is reserved.
Default: 31

but i don't understand why window/2^tcp_app_win
can anyone explain this calculation?

if i have a window of 65000 bytes this means that the buffer for that
packet is 65000/231(default value) = 3.02e-05 bytes ????

sorry but i am confuse.