Manual routing allows you to select specific routing resources for your nets. This gives
you complete control over the routing paths that a signal is going to take. Manual
routing does not invoke
route_design. Routes are directly updated in
the route database.
You might want to use manual routing when you want to precisely control the delay for a net. For example, assume a source synchronous interface, in which you want to minimize routing delay variation to the capture registers in the device. To accomplish this, you can assign LOC and BEL constraints to the registers and I/Os, and then precisely control the route delay from the IOB to the register by manual routing the nets.
Manual routing requires detailed knowledge of the device interconnect architecture. It is best used for a limited number of signals and for short connections.