The /Fo option specifies the name of the object file to be created.
This option is especially useful in combination with the /c option.
Note: there is no space or colon between the /Fo option and the name (which is confusing because the linker option /OUT requires a colon).
Note also, that cl has a -o option to name the object file. However, this option is deprecated (Warning message being Command line warning D9035: option o has been deprecated and will be removed in a future release).
The following Makefile demonstrates the usage of /Fo and /Fe to create obj files and an exe file:
main.c
#include "func.h"
int main() {
func();
return 0;
}