With AC you place a signal in the circuit (by specifying a non-zero value for the mag parameter on a source) and it computes the resulting signal level at any node in the circuit. Generally people set mag=1, which allows them to directly compute the transfer function from that source to any place in the circuit.

With XF you specify the output you are interested in, and Spectre computes the transfer function from every source in the circuit to that output. There are two modes. If you set stimuli=sources (the default) it computes the transfer function from every actual source in the circuit to the output. If you specify stimuli=nodes_and_terminals it computes the transfer function from every place a source could possibly be placed to the output. In other words, it computes a transfer function from every node and selected terminals. The transfer function for a node is the transfer function that would result if you connect a current source from that node to ground. The transfer function for a terminal is the transfer function that would result if you placed a voltage source in series with that terminal.

XF allows you to very quickly measure many transfer functions at once (with a single analysis), like gain, CMRR, PSRR, etc.

-Ken