Here is what Richard Hugo wrote in The Triggering Town:
A poem can be said to have two subjects, the initiating or triggering subject, which starts the poem or “causes” the poem to be written, and the real or generated subject, which the poem comes to say or mean, and which is generated or discovered in the poem during the writing. That’s not quite right because it suggests that the poet recognizes the real subject. The poet may not be aware of what the real subject is but only (has) some instinctive feeling that the poem is done.