Piers Plowman is an allegorical poem by William Langland and except for this one the other three exemplifies the famous epics worldwide where "The Odyssey" is a classical (Greek) epic poem by Homer, "Paradise Lost" is a seventeenth-century epic poem by John Milton and "The Aeneid" is a Latin epic poem by Virgil. The epic poems illustrate a long narrative poem with a vast setting, superhuman characters, supernatural elements, etc. to tell the heroic tales or impart a moral lesson(as Milton to justify the way of Gods) which are culturally significant. Thus, except for the last one all three are epic poems.
Linguist is a person who studies languages, semantics is the science of word and sentence meaning, and arbitrary means that something is subjected to individual will or judgement.
