xf is the trans function analysis. if you assign a node in analysis. you can see the trans function from every input source to this node. The result will be same as the ac analysis if the node and input source are some in both simulation.
pss analysis is used for the period signal circuit. such as VCO
if you do ac analysis, you can see the trans function from one source to every node.
and if you do xf analysis, you can see the trans function from every input source to select node.