Jbuilder : preprocess per_file functionality

I am reading the jbuilder manual(http://jbuilder.readthedocs.io/en/latest/jbuild.html#preprocessing-specification) and am bit confused about how to use preprocess (per_file ...).

From the manual in the build stanza (preprocess (per_file (<spec1> (<module-list1>)) (<spec2> (<module-list2>)) ...))
can <spec1>, <spec2> ... be (pps (ppx_compare ppx_driver.runner))(mod1 mod2) or (action ... stanza ? Are there examples of using those stanzas in per_file if possible?